The report is the second by an investigations team set up by the Organisation for the Prohibition of Chemical Weapons (OPCW), which has the new power to apportion blame for attacks.

The OPCW said in a statement that the Investigations and Identification Team "concludes that units of the Syrian Arab Air Force used chemical weapons in Saraqib on 4 February 2018".

"The report reached the conclusion that there are reasonable grounds to believe that, at approximately 21:22 on 4 February 2018, a military helicopter of the Syrian Arab Air Force under the control of the Tiger Forces hit eastern Saraqib by dropping at least one cylinder," the OPCW report said.

"The cylinder ruptured and released chlorine over a large area, affecting 12 named individuals."

The team issued its first report a year ago, in which it said Syrian President Bashar al-Assad's air force used the nerve agent sarin and chlorine in two attacks on the village of Lataminah in March 2017.