“Syrian aviation made a strike on a large terrorist ammunition depot and a concentration of military hardware in the eastern outskirts of Khan Sheikhoun town,” he said in a video statement, referring to the rebel-held town in northern Syria where the deaths occurred.

He said “chemical-laden weapons” made by rebels at the site had previously been used by militants in Iraq — an apparent reference to a report last month from the International Committee of the Red Cross that toxic agents had been used in fighting near Mosul.

Russia is a key supporter of Assad, who has been fighting rebels trying to unseat him for more than six years. Konoshenkov also claimed chemical munitions had been used by rebels in Aleppo last year.