The United States prepared a rebel force to join the offensive that overthrew the regime of Bashar al-Assad, fighters have claimed.

British and American-trained fighters in the Revolutionary Commando Army (RCA), a group aligned against Islamic State, were told “this is your moment” in a briefing by US Special Forces before Assad was ousted.

In the first indication that Washington had prior knowledge of the offensive, the RCA revealed it had been told to scale-up its forces and “be ready” for an attack that could lead to the end of the Assad regime.

“They did not tell us how it would happen,” Capt Bashar al-Mashadani, an RCA commander, told The Telegraph from a former Syrian army air base used by Russia on the outskirts of the city of Palmyra.

“We were just told: ‘Everything is about to change. This is your moment. Either Assad will fall, or you will fall.’ But they did not say when or where, they just told us to be ready.”