Aleppo rebels agree to UN aid plan

Syrian rebels have signed a deal with the UN for aid delivery in besieged east Aleppo.

Syrian rebels have signed a deal with the UN for aid delivery in besieged east Aleppo. (AAP)

Syrian rebels in besieged east Aleppo have agreed to a UN plan for aid delivery and medical evacuations, but the United Nations is waiting on a green light from Russia and the Syrian government, humanitarian adviser Jan Egeland says.

With freezing winter conditions setting in, some 275,000 people are trapped in east Aleppo, where the last UN food rations were distributed on November 13.

Hundreds of trucks are ready in Turkey and government-controlled west Aleppo to bring aid to the eastern sector, but the United Nations needs 72 hours once it has all approvals to prepare a "big, complex and dangerous operation", Egeland said.

"We do now have written approval in principle by the armed opposition groups of east Aleppo," he told reporters, specifying that he was referring to rebels with whom the UN is in contact - which do not include former Nusra Front militants.

"We have verbal support also from the Russian Federation on our four-point plan. We need written support and we need unconditional support also from Russia and we are waiting still for the answer from the government of Syria."

Hundreds of wounded are awaiting evacuation for treatment under the plan, Egeland said.

Asked about any 'Plan B', he replied: "In many ways Plan B is that people starve. And can we allow that to happen? No, we cannot allow that to happen."