Ahead of a series of summits in Europe on Russia's war in Ukraine, US President Joe Biden warned Wednesday of a "real threat" Russia may use chemical weapons in Ukraine, repeating earlier allegations from the White House.
The last time chemical weapons were unleashed during a conflict was in Syria where a civil war erupted in 2011 as rebels sought to oust President Bashar al-Assad.
Syria only publicly admitted in 2012 that it possessed chemical weapons after pressure from Russia, which was instrumental in working with the United States, to bring Damascus to accept international monitoring and the destruction of its declared toxic arms stocks by the Organisation for the Prohibition of Chemical Weapons (OPCW).
Here is a timeline of chemical weapons use in Syria:
SARIN USE NEAR DAMASCUS
In July 2012 Damascus admits for the first time that it possesses chemical weapons.
Russia's Foreign Minister Sergei Lavrov dismisses Western fears about the regime using those weapons as "rumours and leaks" that should "not be overstated".
In August 2013, the Syrian opposition accuses the government of using the nerve agent sarin in attacks on Eastern Ghouta and Moadamiyet al-Sham, rebel-held areas outside Damascus.
That same month the US blames the regime for the attacks that killed 1,429 people, including 426 children.
In September, a UN report finds "clear and convincing evidence" sarin was used in the attacks.
But US president Barack Obama does not carry out threatened retaliatory strikes, instead reaching a deal with Russia on the dismantling of Syria's chemical arsenal under UN supervision.
A few days later, Putin says that suggesting the regime is involved in the attack is a "provocation".
He claims that rebels had framed the regime for the chemical attack, using "primitive" technology including Soviet-era shells no longer used by the Syrian army.
CHLORINE BARREL BOMBS
In 2016 a UN and OPCW joint commission accuses the regime of dropping chlorine-packed barrel bombs on two villages in rebel-held Idlib province in 2014 and 2015.
Russia immediately questions the report's findings, saying they are not conclusive enough to trigger sanctions.
The Russian ambassador to the UN, Vitaly Churkin, says he has "very serious questions" about the investigations.
"Clearly there is a smoking gun. We know that chlorine was most likely used, but there are no fingerprints on the gun," he says.
The commission also accuses the Islamic State (IS) group of using mustard gas in 2015 in the rebel stronghold of Marea in Aleppo province.
In October 2016, the same joint commission finds the Syrian army carried out a chlorine attack at Qmenas, in the northern Syrian province of Idlib, the year before.
LATAMINAH
In 2018 member states vote in favour of extending the OPCW's powers to determining who is to blame for chemical attacks, instead of simply documenting them.
Russia's deputy foreign minister Sergei Ryabkov says "Moscow does not recognise" the new powers of the global chemical weapons watchdog.
The OPCW accuses the Syrian regime in April 2020 of conducting two chlorine gas attacks on the town of Lataminah, in northern Syria, three years prior.
KHAN SHEIKHUN
In April 2017, warplanes strike the Idlib town of Khan Sheikhun, killing 87 people, including 30 children.
UN and OPCW investigators confirm sarin was used and say Damascus is responsible.
The regime and Moscow deny the accusations.
In response to the attack, US president Donald Trump launches Tomahawk missiles against the regime's Shayrat airbase.
EASTERN GHOUTA
The Syrian Observatory for Human Rights says it suspects the regime used chemical weapons in Eastern Ghouta twice in January 2018, as residents reported breathing difficulties.
In February, a child dies and at least 13 people suffer from breathing problems after another suspected chemical attack on Eastern Ghouta, the war monitor says.
A medic at the scene says he suspects chlorine gas was used.
In March, the Observatory, which has a wide network of sources inside Syria, says at least 60 people suffered breathing difficulties in Eastern Ghouta after regime air strikes.
In April, the White Helmets, who act as first responders in rebel-held areas of Syria, allege regime forces used toxic gas in Douma, killing more than 40.
A week later the US, Britain and France unleash a barrage of guided missiles on suspected regime chemical weapons facilities.
In 2019 the OPCW finds the regime used chlorine gas in a 2018 attack on Douma, near Damascus.
The regime denies involvement.
Russia's deputy foreign minister, Mikhail Bogdanov, says "fabrications and false stories are being used to find some pretext for the use of military force."
NORTHWESTERN SYRIA
In February 2018, residents of Saraqib, south of Aleppo, report a dozen cases of suffocation after regime bombing.
In April 2019, the OPCW says the Syrian air force had used chemical weapons there.
'PLAUSIBLE DENIABILITY'
Moscow repeatedly accused rebels and jihadists of orchestrating chemical attacks.
"False flag warnings about Ukraine blowing up chemical facilities or using chemical weapons and framing Russia are exactly in line with the Syria playbook," said Samuel Ramani, an Associate Fellow at the Royal United Services Institute.
Ramani said Russia has maintained "a narrative of plausible deniability" on the regime using chemical weapons, shifting the blame on the opposition.
Western powers have repeatedly condemned Russia's disinformation campaigns in Syria and Ukraine.
Assistant US ambassador to the UN Security Council Richard Mills said Russia "cannot be trusted when it talks about chemical weapon use in Syria."
