The US has already covertly assisted the Assad government by passing on intelligence about the exact location of jihadi leaders through the BND, the German intelligence service, a source has told The Independent. This may explain why Syrian aircraft and artillery have been able on occasion to target accurately rebel commanders and headquarters.

Syrian army troops are engaged in a fierce battle to hold Tabqa airbase in Raqqa province, the fall of which would open the way to Hama, Syria’s fourth-largest city.

Further north, Isis has captured crucial territory that brings it close to cutting rebel supply lines between Aleppo and the Turkish border. The caliphate declared by Isis on 29 June already covers the eastern third of Syria in addition to a quarter of Iraq. It stretches from Jalawla, a town 20 miles from Iran, which the Iraqi army and Kurdish Peshmerga are trying to recapture, to towns 30 miles north of Aleppo.