Syrian rebels already trained and armed by the US are said to be considering an offer by Russia to switch allegiances from Washington to Moscow in the fight against Isis militants. The leader of the Liwa al-Mu'tasim Brigade, Mustafa Sejry, said he had met a Moscow representative who offered "unlimited amounts of weaponry and close air support" to fight Isis and Jabhat Fatah al-Sham, the al-Qaeda affiliate in Syria.

However his group would have to take its orders from Moscow rather than Washington, which has already invested considerable training and arms in the group.

Sejry said to the Daily Beast he had been told the Kremlin is "more of a friend in fighting Isis" and that "the Americans are not serious about fighting terrorist groups."

However his group is concerned that if it turned towards Moscow, it would join a key ally of their sworn enemies, Assad and his forces.

"Honestly, I would have never ever even thought about working with the Russians after their horrific atrocities against us and their slaughtering thousands of my own people.

But this change of mindset I blame on the Americans," he said, referring to lukewarm American military support. He said he would hold further discussions with the Russians about the proposal.

The revelations come as the UN considers a separate Russian proposal to create humanitarian corridors for civilians who wish to leave Aleppo. Documents seen by the Guardian show that the UN is considering the Kremlin's proposals, despite misgivings by humanitarian agencies.

Moscow suggests creating four areas for humanitarian corridors for civilians to flee eastern Aleppo, but aid agencies are concerned that Assad and his allies could use the plan to justify a more indiscriminate campaign against eastern Aleppo.

In addition, aid workers say the fighting in the region would make the proposed corridors unworkable, the paper said.

President Bashar al-Assad's forces and Iranian-backed militias have besieged Eastern Aleppo, which is under opposition control.

On Thursday (4 August), a Syrian group said it would exchange the bodies of five people, who were shot down in a Russian military helicopter, if prisoners held in Syrian and Lebanese government jails are freed.