The UN Organisation for the Prohibition of Chemical Weapons (OPCW) has said that chlorine gas was used in an attack on Syria's northern rebel-held town of Kafr Zeita on 1 October, 2016.
The watchdog said that an investigation, including witness interviews and digital evidence, showed that two industrial chlorine cylinder barrels were used "near a field hospital" and that "there are reasonable grounds to believe that [they] were used as a weapon".
The report quoted witnesses as saying that a helicopter took off from the regime-held Hama airport before the attack on an agricultural area where a number of rebel groups were sheltering in caves.
"Shortly afterwards, the helicopter dropped two barrels, according to a number of witnesses, while others reported being aware of one barrel only," the report said. "Approximately 20 individuals suffered from suffocation and breathing difficulties."
The organisation pointed out that it had obtained one of the cylinders used in the attack. "The industrial cylinder was engraved with markings reading Cl2, the molecular formula for chlorine gas."
The OPCW report will now be submitted to the UN Security Council.
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