The world chemical weapons watchdog has said Syrian forces carried out a chlorine gas attack on Douma in 2018 that killed 43 people.
The attack was part of efforts to remove rebels from the outskirts of Damascus in the decade-long war.
The Organisation for the Prohibition of Chemical Weapons said a Syrian army helicopter had dropped toxic chlorine gas cylinders on civilian areas.
The Syrian government has denied using chemical weapons.
The OPCW said there were “reasonable grounds” to conclude that the Syrian Air Force had perpetrated the chemical weapons attack on Douma on 7 April 2018.
“At least one” helicopter of the elite Syrian “Tiger Forces” unit, it said, dropped two yellow cylinders containing toxic chlorine gas on two apartment buildings in a “civilian-inhabited area” in Douma, 10km (six miles) north-east of Damascus.
The first cylinder “ruptured, and rapidly released toxic gas – chlorine – in very high concentrations, which rapidly dispersed within the building”, killing 43 people, the OPCW said.
The second cylinder smashed into an apartment and slowly released some chlorine “mildly affecting those who first arrived at the scene”.
“The use of chemical weapons in Douma – and anywhere – is unacceptable and a breach of international law,” said OPCW Director-General Ambassador Fernando Arias.
The report was based on 70 environmental and biomedical samples, 66 witness statements and other data including forensic analysis, satellite images, gas dispersion modelling and trajectory simulations, OPCW said.
A previous OPCW investigation concluded in March 2019 that a chemical attack had taken place in Douma, but it was not tasked with assigning blame.
Around the time of the attack, Syrian President Bashar al-Assad accused the US, UK and France of “staging” the incident in Douma with the help of the Syria Civil Defence, also known as the White Helmets.
“It was a lie. After we liberated that area our information confirmed the attack did not take place,” he told the Daily Mail in June 2018.
Syria’s ally Russia has also claimed the attack was staged, describing it as part of a “Russophobic campaign”.
A British diplomat called the allegation “bizarre” and a “blatant lie”.
A peaceful uprising against President Assad that started in early 2011 turned into a full-scale civil war in Syria that has seen hundreds of thousands of civilians and combatants killed.
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