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 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.
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 Damascus in chemical weapons strikes after Syrian troops launch an offensive in the area.
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 -
In September 2014 the Organisation for the Prohibition of Chemical Weapons (OPCW) says chlorine was used as a weapon "systematically and repeatedly" on villages in the central province of Hama and northwestern Idlib province.
A joint UN-OPCW commission finds in 2016 that helicopters from regime-controlled air bases dropped chlorine barrel bombs on villages in Idlib in 2014 and 2015.
The commission accuses the jihadist Islamic State group of using mustard gas in August 2015 in the rebel stronghold of Marea in northern Aleppo.
In October the commission says that the Syrian army carried out a chlorine attack in Idlib province in March 2015.
- New sarin attack -
Warplanes strike the rebel-held town of Khan Sheikhun in Idlib in April 2017 with a chemical agent. More than 80 were killed, according to the UN and Syrian Observatory for Human Rights monitor.
The OPCW later says sarin gas was used but does not assign blame.
UN war crimes investigators say later 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".
- Russian veto -
In November 2017 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 the Observatory says at least 21 people, including children, suffered breathing difficulties in a suspected Syrian regime chemical attack in a besieged rebel enclave near Damascus.
A similar attack had targeted the outskirts of the same city, Douma, in the opposition-held Eastern Ghouta region on January 13, according to the monitor, which says seven people suffered breathing problems.