Rebel-held eastern Aleppo in northern Syria may face "total destruction" in two months, with thousands killed, the UN's Syria envoy has said.

Staffan de Mistura told reporters that he was prepared to personally accompany al-Qaeda-linked jihadists out of the city if it would stop the fighting.

He also appealed to Russia and Syria not to destroy the city for the sake of eliminating militants.

The UN says 275,000 civilians are trapped in the war-torn east of Aleppo.

"The bottom line is, in a maximum of two months... the city of eastern Aleppo at this rate may be totally destroyed," Mr de Mistura told a news conference in Geneva.

"Thousands of Syrian civilians, not terrorists, will be killed and many of them wounded."

He added: "This is what you, we, the world will be seeing when we will be trying to celebrate Christmas, or the end of the year, if this continues at this rate, unimpeded. Homs [Syrian city] multiplied by 50."

He said the UN was not willing to "be passive, resign ourselves" to genocides such as those in Srebrenica and Rwanda.

'Leave in dignity'

Russia and Syria say their forces are attacking the powerful Jabhat Fateh al-Sham (formerly known as al-Nusra Front) in eastern Aleppo.

Mr de Mistura said the group was about 900 strong in eastern Aleppo and he would willingly accompany them out of the city if they decided to leave.

Addressing the group directly, he asked if they were willing to decide the destiny of 275,000 people.

"If you did decide to leave in dignity, and with your weapons, to Idlib or anywhere you wanted to go, I personally am ready physically to accompany you," he said.

He also warned Syria and Russia that history would judge them if they used the presence of the jihadists in Aleppo as "an alibi perhaps for destroying a whole city".

The besieged east of Aleppo has come under intense aerial bombardment since a cessation of hostilities brokered by the US and Moscow collapsed last month.

Diplomatic efforts to revive the ceasefire have so far come to nothing.

The US has since suspended any military co-operation with Russia over Syria.