Since the Syrian conflict began in March 2011, multiple players have accused each other several times of using chemical weapons.

Here is a recap of the situation.

- Damascus threatens to use chemical weapons -

- July 23, 2012: The Syrian government acknowledges for the first time that it has chemical weapons and threatens to use them in the event of military operations by Western countries, but not against its own population.

On August 20, US President Barack Obama says that using or even moving such weapons would constitute the crossing of a "red line."

- Sarin gas attack near Damascus -

- August 21, 2013: Hundreds of people are killed in the east and southwest of Damascus, including in the neighbourhood of Moadamiyet al-Sham, in chemical weapons strikes after Syrian troops launch an offensive in the area.

The opposition accuses the Syrian army, but the government denies it.  

In late August, a US intelligence report blames the Syrian regime for the Moadamiyet al-Sham attack and adds that 426 children were among 1,429 people killed.
On September 16, a UN reports says there is clear evidence that sarin gas was used on August 21.

Two days earlier however, Washington and Moscow agree on a plan to eliminate Syria's chemical weapons by the middle of 2014, putting off the threat of punitive strikes against the Assad regime by Washington and Paris.

- Chlorine attacks in northern and central Syria -

- September 10, 2014: Investigators from the Organisation for the Prohibition of Chemical Weapons (OPCW) confirm that chlorine was used as a chemical weapon "systematically and repeatedly" in villages in northern Syria earlier in the year. It cites attacks in the villages of Talmanes, Al-Tamana and Kafr Zeita.

The watchdog group Human Rights Watch says that attacks on those villages in April were the work of Syrian government forces.

In late August, an UN commission accuses Syrian authorities of using chemical weapons, probably chlorine, eight times in western Syria.

- August 7, 2015: The UN Security Council agrees to form a panel of experts to determine who was responsible for chlorine attacks in Syria.
Washington, London and Paris accuse the Syrian army, but Moscow says there is no irrefutable evidence that was the case.

- IS accused of using mustard gas -

- August 25, 2015: Syrian rebels and non-governmental organisations say they documented a chemical weapons attack against dozens of people on August 21 in Marea, the main rebel stronghold in the northern Aleppo province. Local activists and the Syrian Observatory for Human Rights accuse the Islamic State (IS) group.

MSF, a Paris-based medical humanitarian group, said it treated four patients, all from a single family in Marea, who were "exhibiting symptoms of exposure to chemical agents" in northern Syria on August 21.

The Syrian American Medical Society (SAMS) says its doctors have identified the agent as mustard gas.

- November 5, 2015: An OPCW source tells AFP that mustard gas was used in Marea on August 21, saying: "We have determined the facts, but we have not determined who was responsible."

Source: AFP