BEIRUT: Since the start of the conflict in Syria in 2011, the belligerents -- in particular the regime of President Bashar al-Assad -- have been accused on numerous occasions of using chemical weapons.
Here is a summary.
- Damascus threatens -
The Syrian government acknowledges in July 2012 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.
The following month, US President Barack Obama says the use or even movement of such weapons would be a "red line" for his administration.
- Sarin attack -
In August 2013, hundreds of people are killed in the suburbs of Damascus in chemical weapons strikes after Syrian troops launch an offensive there.
The opposition blames the regime, which denies involvement.
In late August, a US intelligence report says with "high confidence" that Syria's government carried out the attacks. It says 1,429 people were killed, including 426 children.
A UN report says later there is clear evidence sarin gas was used.
The following month, the United States and regime-backer Russia reach a deal to eliminate Syria's chemical weapons by 2014, averting punitive US strikes.
- Chlorine, mustard gas -
A joint commission of the United Nations and Organisation for the Prohibition of Chemical Weapons (OPCW) in August 2016 finds that helicopters from regime-controlled air bases dropped chlorine barrel bombs on villages in the northwestern province of Idlib in 2014 and 2015.
The commission accuses the Islamic State jihadist group of using mustard gas in August 2015 in the rebel stronghold of Marea in the northern province of Aleppo.
In October, the commission says the Syrian army carried out a chlorine attack in Idlib province in March 2015.
Of the nine suspected chemical attacks investigated by the commission, from 2014 and 2015, three are attributed to the Syrian regime and one to IS.
- New sarin attack -
Warplanes strike the rebel-held town of Khan Sheikhun in Idlib in April 2017 with a chemical agent. More than 80 are killed, the United Nations and Syrian Observatory for Human Rights monitor say.
The OPCW later says sarin gas was used but does not assign blame.
UN war crimes investigators later say they have evidence that Syrian government forces were responsible, allegations rejected by Moscow.
In November 2017, Western governments say a presumed chemical attack which just preceded that in Khan Sheikhun "bears the hallmarks of the Syrian regime".
Russia twice casts its veto at the UN Security Council to block the extension of the UN-led investigation to determine who is behind chemical weapons attacks in Syria.
- Damascus accused again -
On January 22, 2018, at least 21 people, including children, suffer breathing difficulties in a suspected regime chemical attack in a besieged rebel enclave near Damascus, the Observatory says.
A similar attack had targeted the outskirts of the same city, Douma, in the opposition-held Eastern Ghouta region on January 13, it says.
On February 4, eleven people are treated for "suffocation" amid a "foul smell" after regime air strikes on the northwestern town of Saraqeb, the Observatory says.
On February 6, UN war crimes investigators say they are probing suspected chemical weapons use in Eastern Ghouta and Saraqeb.