By Stephanie Nebehay

GENEVA (Reuters) - Syrian rebels in besieged east Aleppo have agreed to a U.N. plan for aid delivery and medical evacuations, but the United Nations is awaiting a green light from Russia and the Syrian government, humanitarian adviser Jan Egeland said on Thursday.

With freezing winter conditions setting in, about 275,000 people are trapped in east Aleppo, where the last U.N. food rations were distributed on Nov. 13.

Hundreds of trucks are ready in Turkey and government-controlled west Aleppo to bring food and medicines 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 U.N. is in contact - and 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."

Egeland hoped the plan, which includes rotation of the 30 doctors still in eastern Aleppo, can be carried out "in the next few days".

The siege and intense bombardment of east Aleppo, aggravated by renewed, frequent air strikes on hospitals in the past week, have left residents even shorter of medicines, food and fuel.

Major and regional powers discussed the need to protect medical facilities, Egeland said.

"We want to try to launch a system that could get out of this horrific situation that medical facilities, clinics, hospitals are attacked again and again and again," he said.

Medical facilities should be clearly marked, used only for civilian purposes, and their locations sent "to all of the military actors that are using air warfare to avoid any more attacks", he said.

Hundreds of wounded await 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."

An estimated 974,000 Syrians live in besieged areas, including 850,000 encircled by government forces and the rest trapped by Islamic State militants and other rebels, the U.N. says.

Egeland, referring to government-besieged Madaya and Zabadani near the Lebanon border, and the villages of Foua and Kefraya encircled by rebels in Idlib, said: "We do hope to be able to go to the '4 towns' this weekend and it is urgent."

(Reporting by Stephanie Nebehay; editing by Mark Heinrich and Janet Lawrence)