Mr Kerry made clear that America could accept a Syrian peace settlement that left Assad in power for an interim period. “We’re not being doctrinaire about the specific date or time [for Assad’s departure] – we’re open,” said Mr Kerry.

“It doesn’t have to be on day one or month one. There is a process by which all the parties have to come together and reach an understanding of how this can best be achieved.”

Mr Kerry noted that Isil and other extremists emerged from Assad’s campaign against Syria’s largely Sunni population. By intervening directly in Syria, Russia may allow an escalation of this offensive – and end up boosting Isil.

Mr Kerry said that was a “lack of logic” in Russia’s position that “they are bringing in more equipment to shore up Assad at the same time they say they are going after” Isil.