United Nations chemical weapons inspectors were already in the country, having arrived in Syria on 18 August 2013 to carry out a 2-week mission in Syria to investigate earlier incidents in which the Syrian government and rebels accused each other of carrying out chemical attacks. The inspectors' mandate was only to try to establish whether chemical weapons were used at all, not who used them. The mission was also limited to investigating the alleged use of chemical weapons in 3 areas, including a March attack in the Aleppo suburb of Khan al-Assal. The UN had continued to push for unfettered access for the inspectors and western powers called for the team to investigate the new allegations. The UN Security Council subsequently called for a clarification of the incident and "a thorough, impartial and prompt investigation" of the incident. On 22 August 2013, France's foreign minister Laurent Fabius took the hardest line among European powers, saying that "If [the attack] is proven, France's position is that there must be a reaction, a reaction that could take the form of a reaction with force" and that "If the Security Council cannot take a decision, at that moment decisions must be taken in another way," implying the potential for a unilateral use of force. China and Russia remained staunchly opposed to the use of force and NATO members Germany and Turkey both called for increased sanctions, but refused to speculate on the potential for the use of force.

On 25 April 2013, during a trip to the United Arab Emirates, US Defense Secretary Chuck Hagel said that the US intelligence community believed the Syrian government had used sarin gas on a small scale against rebels trying to overthrow President Bashar al-Assad. Also on 25 April 2013, the White House delivered a letter to two members of Congress on the topic of chemical weapons use in Syria, saying that US intelligence had confirmed the information "to some degree of varying confidence."

On 22 December 2012 Russian Foreign Minister Sergei Lavrov said that Syrian chemical weapons are under the control of the Syrian government, which has consolidated them in one or two locations. “As of right now… the [Syrian] government is doing all it can to safeguard those weapons,” he said, adding that “we are following all leads concerning chemical weapons.” However, there is a potential danger those weapons could be seized by militants, he admitted.

On 12 July 2012 news reports indicated that some Syrian chemical weapons were being moved from their storage areas. Some US officials were concerned that the weapons might be used against rebels or civilians, or the weapons were possibly being hidden from the opposition or Western powers. On 13 July 2012 Pentagon Press Secretary George Little told reporters there were no indications that Syrian chemical weapons stockpiles have become less secure. “Our assessment remains what it’s been for some time. The Syrian regime has control of its chemical weapons stockpiles,” Little said at the Pentagon.

“We believe that the Syrian government has a very serious responsibility to protect its stockpiles of chemical weapons,” he added. “We would, of course, caution them strongly against any intention to use those weapons. That would cross a serious red line.” If any Syrian officials choose to utilize chemical weapons they will be held accountable for their actions, he said. “We have a Syrian regime that continues to perpetrate multiple attacks against the civilian population every day ... If they were to take further steps, it would be an even graver situation,” Little said.

Syria may only use chemical weapons against external aggression, Syrian Foreign Ministry spokesman Jihad Makdissi said on 23 July 2012. "Syria may use chemical weapons only in case of foreign aggression," according to a Foreign Ministry statement read by Makdissi during a press conference in the Syrian capital Damascus. Syria will never use chemical weapons against civilians, he added, stressing that the weapon stocks are secure.

U.S. President Barack Obama warned the Syrian authorities that they would be held accountable if they decide to use their arsenal of chemical weapons. "Given the regime's stockpiles of chemical weapons, we will continue to make it clear to Assad and those around him that the world is watching and that they will be held accountable by the international community and the United States should they make the tragic mistake of using those weapons," Obama said on 24 July 2012.

The Syrian authorities assured Moscow that there will be no use of chemical weapons against rebel forces, Russia’s foreign minister said on 06 November 2012. “I rule out the use by the [Syrian] regime of chemical weapons,” Foreign Minister Sergei Lavrov told journalists. “We have received the appropriate assurances.” Lavrov said Russia had also asked Syria to make a similar pledge to Western powers.

Syria, not a signatory to either the CWC or the Comprehensive Nuclear Test Ban Treaty (CTBT), and not having ratified the BWC, began developing chemical weapons in 1973 prior to the Yom Kipper war when the Egyptian government reportedly gave Syria artillery shells capable of delivering chemical weapons. Since then Syria has developed a robust chemical weapons program, perhaps one of the most advanced in the Middle East, and a variety of delivery methods. The country is still very depending on outside assistance in procuring important precursor chemicals and equipment.

Apparently, during the early 1980's, a policy decision within the Syrian government, led to the rapid development of non-conventional weapons. It is likely this decision was the result of the realization that Syria could not reach conventional military parity with Israel.

The US Government reported August 30, 2013 that "We assess with high confidence that the Syrian regime has used chemical weapons on a small scale against the opposition multiple times in the last year, including in the Damascus suburbs. This assessment is based on multiple streams of information including reporting of Syrian officials planning and executing chemical weapons attacks and laboratory analysis of physiological samples obtained from a number of individuals, which revealed exposure to sarin. We assess that the opposition has not used chemical weapons."

On 09 September 2013 US Secretary of State John Kerry, in what seemed like an offhand remark this past week, mentioned chemical weapons disarmament as a possible solution to the Syria impasse. Moscow picked up on the remark almost immediately and offered it as a proposal and the Syrians, again almost immediately, accepted it. Most experts dismiss suggestions that this can be done easily or quickly, and certainly not within the tight timetables being mentioned by some diplomats. It wwould take many months, at least, to transfer the entire arsenal out.

Syrian President Bashar al-Assad’s acceptance in principle of a Russian proposal to place his deadly chemical weapons under international supervision is likely facing resistance from his own military, analysts said. But Assad may have had little choice because of Iranian and Russian pressure.

Syrian rebels who had been battling to oust Assad for more than two years dubbed the disarmament offer a “cheap trick.” The opposition Syrian National Coalition argued that it is a “political maneuver that will lead to pointless procrastination and will cause more death and destruction to the people of Syria.” Without a cease-fire, the danger could be too great for weapons inspectors, most likely drawn from the Organization for the Prohibition of Chemical Weapons, the international body overseeing the Chemical Weapons Convention.

The fact-finding mission (FFM) appointed by the OPCW Director-General to examine alleged uses of chlorine gas as a weapon in Syria reported 10 September 2014 that it had found information constituting “compelling confirmation” that a toxic chemical was used “systematically and repeatedly” as a weapon in villages in northern Syria earlier in 2014. In its second report that included key findings, the Mission stated that “the descriptions, physical properties, behaviour of the gas, and signs and symptoms resulting from exposure, as well as the response of patients to the treatment, leads the FFM to conclude with a high degree of confidence that chlorine, either pure or in mixture, is the toxic chemical in question.” Following the establishment of the FFM in late April 2014, there was a marked reduction in reported chlorine attacks in the months of May, June and July. But there was a spate of new allegations in August.

Although toxic, chlorine is not considered a chemical weapon. Chlorine was not on the list of chemicals Syria had to declare as part of a chemical disarmament agreement in 2013. During the Great War, chlorine and phosgene gases were released from canisters on the battlefield and dispersed by the wind. The first large-scale attack with chlorine gas occurred 22 April 1915 at Ieper in Belgium.

The OPCW Director-General condemned Chlorine attacks in Iraq in a statement issued 23 February 2007. "As Director-General of the Organisation for the Prohibition of Chemical Weapons, I condemn in the strongest possible terms, the recent multiple use of chlorine gas by groups in Iraq to kill and injure innocent civilians. The international community has firmly rejected the use of toxic chemicals under any circumstances to inflict harm..."

Human Rights Watch said April 14, 2015 that its investigations into attacks by Syrian government forces in March 2015 in Idlib province "strongly suggest" the military dropped barrel bombs with toxic chemicals. The group said that based on reports from witnesses as well as audio and video from attack sites that there were strong indications of chemical attacks at three sites, while three others needed further investigation.

The UN Security Council unanimously agreed August 07, 2015 to establish a joint investigation mechanism to identify perpetrators of chlorine and other toxic chemical attacks in Syria. It is the first time the council has sought to establish accountability for such attacks in Syria. The Organization for the Prohibition of Chemical Weapons (OPCW) said its monitors were highly confident that the toxic chemical chlorine had been used repeatedly and systematically as a weapon. Chlorine has been dropped in barrel bombs from aircraft. The Syrian opposition does not have any planes or helicopters, only the government.

Accusations of chemical attacks have been a fixture of the Syrian conflict with both the government and rebels blaming the other during the past five years. International inspectors issued a report in August 2016 saying government forces and Islamic State militants had each carried out chemical attacks. United Nations experts said more investigations were ongoing into alleged chemical attacks earlier this year, including in Aleppo.

At least 70 people were treated for breathing problems 06 September 2016 in the Syrian city of Aleppo after what rescue workers say was a chlorine gas attack by government forces. The Syria Civil Defense group said helicopters dropped several barrels containing chlorine on the opposition-held al-Sukkari neighborhood. The Britain-based Syrian Observatory for Human Rights also reported a barrel bomb attack in the area, but could not confirm whether chlorine was involved.

New York-based Human Rights Watch accused the Syrian government of mounting eight chlorine gas attacks on insurgent-controlled areas during the final weeks of the battle for Aleppo. A joint investigation by the United Nations and international chemical weapons experts blamed the Syrian government for at least three chlorine attacks in 2014 and 2015.

On 12 January 2017 the U.S. Department of the Treasury’s Office of Foreign Assets Control (OFAC) took action in response to the Organization for the Prohibition of Chemical Weapons (OPCW) - United Nations (UN) Joint Investigative Mechanism (JIM) findings that the Syrian regime used industrial chlorine as a weapon against its own people. OFAC designated 18 senior regime officials connected to Syria’s weapons of mass destruction program and identifying five Syrian military branches as part of the Government of Syria.

In reports issued in August and October 2016, the JIM – established by the UN Security Council to investigate incidents of already-confirmed chemical weapon attacks – found that the Syrian government, specifically the Syrian Arab Air Force, was responsible for three chlorine gas attacks in Talmenes on April 21, 2014, and in Qmenas and Sarmin on March 16, 2015. In response to the reports’ findings, OFAC for the first time sanctioned Syrian military officials in connection with the Syrian regime’s use of chemical weapons.

“The Syrian regime’s use of chemical weapons against its own people is a heinous act that violates the longstanding global norm against the production and use of chemical weapons,” said Adam J. Szubin, Acting Under Secretary for Terrorism and Financial Intelligence. “Today’s action is a critical part of the international community’s effort to hold the Syrian regime accountable for violating the Chemical Weapons Convention (CWC) and UN Security Council Resolution 2118.”

As a result of this action, any property or interest in property of the identified persons in the possession or control of U.S. persons or within the United States must be blocked. Additionally, transactions by U.S. persons involving these persons are generally prohibited.

The UN Security Council held an emergency meeting 05 April 2017 to discuss a chemical weapons attack in Syria that has killed scores of people, amid a continuing global chorus of condemnation over the incident. Observers, diplomats and analysts blamed the incident in the town of Khan Sheikhoun on forces loyal to President Bashar al-Assad and his Russian allies. The World Health Organization says the victims appear to show symptoms consistent with exposure to a nerve agent. An umbrella organization of Western relief groups put the death toll at 100 with more than 350 others stricken.

Russia "categorically denied" any role in the attack, blaming the incident on Syrian warplanes striking a warehouse or factory storing chemical weapons possessed by rebel forces. Syria's government has repeatedly denied using chemical weapons against civilians throughout the six-year war.

The reported attack on Khan Sheikhoun to the west of Aleppo and south of the city of Idlib, the provincial capital, is the third claimed chemical weapons attack in just over a week in Syria. Another two blamed on the Syrian government were reported in Hama province, not far from Khan Sheikhoun.

Human Rights Watch accused Syrian government forces of using deadly nerve gas on four occasions in recent months, including the April 4 chemical attack on Khan Sheikoun that killed nearly 100 people. In a report issued 01 May 2017, the rights group said forces loyal to Syrian leader Bashar al-Assad also carried out gas attacks in December of 2016 and March of 2017. “The government’s recent use of nerve agents is a deadly escalation – and part of a clear pattern,” said Kenneth Roth, executive director of Human Rights Watch. “In the last six months, the government has used warplanes, helicopters, and ground forces to deliver chlorine and sarin in Damascus, Hama, Idlib, and Aleppo. That’s widespread and systematic use of chemical weapons.”

Syrian opposition activists and rescuers said that a poison gas attack 01 April 2018 on a rebel-held town near the capital has killed at least 40 people, allegations denied by the Syrian government. First responders said they discovered families suffocated in their homes and shelters with foam on their mouths. Relief workers said more than 500 people, mostly women and children, were brought to medical centers with difficulty breathing, foaming at the mouth and their eyes burning. The Civil Defense and Syrian American Medical Society said patients gave off a chlorine-like smell, and some had blue skin, an indication of oxygen deprivation.

The Russian foreign ministry rejected claims of a chemical attack, saying, "The spread of bogus stories about the use of chlorine and other poisonous substances by [Syrian] government forces continues. "We have warned several times recently against such dangerous provocations," the Moscow statement said. "The aim of such deceitful speculation, lacking any kind of grounding, is to shield terrorists and to attempt to justify possible external uses of force."

"The declaration of use has become a boring and ineffective process, with the exception of several countries that trade blood and support terrorism in Syria," the Syrian National Agency quotee the Foreign Ministry of Syria. The ministry stressed that reports of chemical weapons are emerging every time the Syrian army launches a successful operation against terrorists and also to extend the life of terrorists in the city of Duma in eastern Ghuda around Damascus. "The use of chemical weapons in Ghuda by terrorist groups was planned, information on this was credible and confirmed, Syrian leadership warned against it," they said at the Foreign Ministry of Syria.

The U.S. State Department accused the Syrian government of carrying out a new chemical weapons attack in the northwestern region of the country on Sunday. State Department Spokeswoman Morgan Ortagus said in a statement on Tuesday 21 May 2019 : “We are still gathering information on [Sunday’s] incident, but we repeat our warning that if the Assad regime uses chemical weapons, the United States and our allies will respond quickly and appropriately.”

Ortagus then shifted her attention to Russia's accusations that rebel groups in Idlib were planning to stage a new chemical weapons attack. She accused the Russian government of "continuing disinformation campaign ... to create the false narrative that others are to blame for chemical weapons attacks ... The facts, however, are clear... The Assad regime itself has conducted almost all verified chemical weapons attacks that have taken place in Syria – a conclusion the United Nations has reached over and over again.”