Russia on Tuesday vetoed a US-drafted resolution that would have extended the mandate of a panel investigating who is behind chemical weapons attacks in Syria.

Here is a recap of chemical strikes and related events since the conflict began in March 2011.

- 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 the east and southwest of 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 against the regime.

- Chlorine -

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 northwestern Syria's rebel-held Idlib province earlier.

Human Rights Watch accuses Syrian government forces. 

The UN Security Council agrees in August 2015 to form a panel to investigate chlorine attacks in Syria.

Washington, London and Paris blame the Damascus regime for such attacks, but Moscow says there is no irrefutable evidence.

A UN commission finds in 2016 that helicopters from regime-controlled air bases dropped chlorine barrel-bombs on villages in Idlib in 2014 and 2015.

In late August the commission accuses Syrian authorities of using chemical weapons, probably chlorine, eight times in the west of the country.

- IS accused of mustard gas attacks -

Syrian rebels and activists report a chemical weapons attack in August 2015 in the rebel stronghold of Marea in northern Aleppo, affecting dozens of people.

Locals and the Britain-based Syrian Observatory for Human Rights accuse the Islamic State group.

Doctors Without Borders (MSF) says it treated four civilians for "symptoms of exposure to chemical agents" and witnesses talk of a "yellow gas".

A 2016 UN investigation blames IS, saying it is "the only entity with the ability, capability, motive and the means to use sulphur mustard".

In October the UN-led Joint Investigative Mechanism says that of nine alleged chemical attacks it is examining, three can be attributed to the Syrian government and one to IS jihadists.

- Russian-Chinese veto -

In February 2017 Russia and China block a Western-backed UN resolution that would have imposed fresh sanctions on the Syrian regime over chemical weapons use.

The following month the OPCW says it is examining allegations of eight toxic gas attacks in Syria since the beginning of the year.

- New sarin attack -

Warplanes strike the rebel-held town of Khan Sheikhun in Idlib in April 2017 with a chemical agent, killing at least 83 people. The OPCW later says sarin gas was used but does not assign blame.

United Nations war crimes investigators say later they have evidence that Syrian government forces were responsible, allegations that are denied by the regime.