The investigation from the UN and the Organisation for the Prohibition of Chemical Weapons (OPCW), released on Thursday, said that experts are “confident that the Syrian Arab Republic is responsible for the release of sarin at Khan Sheikhoun on 4 April 2017”.
Dozens of people were rushed to hospital in the rebel-held town in Idlib province after the incident with symptoms such as convulsions, suffocation, coughing blood and foaming at the mouth, consistent with either sarin gas - a nerve agent - or chlorine exposure, Medecins Sans Frontiéres said their staff found.
Images and video from the incident, including dying children, caused outrage around the world, and eventually led to US President Donald Trump’s "warning shot” barrage of 59 Tomahawk missiles on a nearby Syrian airbase – the only direct US intervention in Syria’s civil war to date.
Mr Assad has repeatedly denied his government has any chemical weapons stocks after agreeing to give them up to international monitors in 2013. Damascus, along with its Russian and Iranian allies, have said that the casualties were caused when a conventional air strike on an al-Qaeda weapons depot nearby caused an explosion, releasing the deadly gases.
Russia has used its position as a permanent member of the UN Security Council to repeatedly shield the Syrian government from international action. Earlier this week, Moscow vetoed a resolution extending the mandate of the UN and OPCW’s Joint Investigative Mechanism (JIM) into the use of chemical weapon’s in Syria’s war.
The new JIM report, however, backs up preliminary findings by UK, US, French and Israeli intelligence that it was a Syrian warplane that dropped sarin on the town.
“Britain condemns this appalling breach of the rules of war and calls on the international community to unite to hold Assad's regime accountable,” Foreign Secretary Boris Johnson said in response to the report.
His comments were echoed by US ambassador to the UN, Nikki Haley, who also reprimanded Russia for shielding the Syrian government. “In spite of these independent reports, we still see some countries trying to protect the regime. That must end now,” she said. “It is the security council’s role to make it clear that “the use of chemical weapons by anyone will not be tolerated.”
A statement from the Russian mission to the UN said “We have started a thorough study of this paper, which is of very complex technical nature.“
There has been no formal response from the Syrian authorities.
The Syrian government surrendered its chemical arsenal to the OPCW after the deaths of hundreds in a sarin attack on a rebel-held suburb of Damascus in 2013, although the opposition has long accused the regime of holding back some of its supply.
The international watchdog has said it believes Mr Assad's government is responsible for at least three chemical weapons attacks that have occurred in Syria since then - claims the president also denies.
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