“Multiple government helicopters were observed over Douma on April 7, with witnesses specifically reporting a Mi-8 helicopter, known to have taken off from the Syrian regime’s nearby Dumayr airfield, circling over Douma during the attack,” the statement said. “Numerous eyewitnesses corroborate that barrel bombs were dropped from these helicopters, a tactic used to target civilians indiscriminately throughout the war. Photos of barrel bombs dropped in Douma closely match those used previously by the regime.”

Based on that evidence, the White House assessment concluded: “These barrel bombs were likely used in the chemical attack.”

The statement said that “a significant body of information” pointed to the use of chlorine in the Douma attack while “some additional information” indicated that the nerve agent sarin was also used. The defence secretary, James Mattis, said on Friday evening the US was sure about chlorine use in Douma but was “not certain about sarin right now”.