Syrian President Bashar Assad was required to declare and dispose of all his chemical weapons under UN supervision in 2014, but his forces have been accused of using them since then, most recently this week, when a chemical attack in northern Syria killed nearly 90 people.
Syria's disarmament, which was carried out amid a chaotic civil war, has been the subject of some doubt, and there is evidence that the Islamic State group and other insurgents have acquired chemical weapons. Here's what we know and don't know about chemical weapons in Syria, reports the Associated Press.
A chemical weapons attack in the Damascus suburbs in August 2013 killed hundreds of people and sparked worldwide outrage. The United States blamed Assad's forces and threatened punitive strikes, but backed off at the last minute following a surprise agreement with Moscow to remove all of Syria's previously undisclosed chemical weapons.
In September, The UN Security Council ordered Syria to account for and destroy its chemical weapons stockpile, and threatened to authorise the use of force in the event of non-compliance. In October, Syria became a signatory to the Chemical Weapons Convention, prohibiting it from producing, stockpiling, or using chemical weapons.
Ahead of disarmament, Assad's government disclosed it had some 1,300 tons of chemical weapons, including sarin, VX nerve agent and mustard gas.
The entire stockpile was said to have been dismantled and shipped out under international supervision in 2014. The chemical weapons were shipped outside Syria and destroyed abroad, with the most toxic material disposed of at sea aboard a US ship.
The Organisation for the Prohibition of Chemical Weapons (OPCW), then-Secretary of State John Kerry and others said the entire stockpile had been removed and destroyed. This did not include chlorine, which has civilian uses.
A year later, diplomats at the United Nations said experts had found traces of deadly nerve agents used to make chemical weapons at a site in Syria. The discovery of traces of sarin and VX renewed fears that Syria did not disclose all aspects of its chemical weapons program when it joined the Chemical Weapons Convention in 2013.
The OPCW said in April 2015 that members of the body have raised questions about Syria's declaration of complete disarmament. It said its technical secretariat is engaged with Syrian authorities to clarify gaps, inconsistencies and discrepancies.
Soon after the declaration that everything was destroyed, reports began emerging of chlorine attacks and the use of other suspected chemical weapons agents. Few seemed shocked by the prospect that Assad's government had secretly held onto some of its stash.
No one knows for certain what the government might still have hidden away. And even if they did, it would be difficult to do anything about it. No one can enter government-held territory without its consent, and trying to destroy suspected stockpiles with airstrikes could disperse the chemical agents, with potentially catastrophic consequences.
Syria and its close ally Russia say that Islamic State and al-Qaeda-linked militants in Syria have acquired chemical weapons. Russia's Defence Ministry said this week's attack in the northern town of Khan Sheikhoun, which killed 87 people, was caused by Syrian airstrikes hitting a weapons warehouse where al-Qaeda-linked fighters were storing chemical agents.
Insurgents may have acquired chemical weapons by seizing government military bases or by trucking them across the border from Iraq. The UN says the Islamic State group has used crude chemical weapons in both Syria and Iraq.
Khan Sheikhoun is in the northern Idlib province, which is dominated by the al-Qaeda-linked Fatah al-Sham Front but far from any territory controlled by IS.
Assad supporters have floated unsubstantiated theories that warplanes from other countries may have dropped chemical agents on Khan Sheikhoun, or that al-Qaeda may have detonated a car bomb filled with chemicals.