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

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

Hundreds of trucks were ready in Turkey and government-controlled west Aleppo to bring food and medicines to the eastern sector, but the UN needed 72 hours once it had all approvals to prepare a “big, complex and dangerous operation”, Egeland said on Thursday.

“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, who 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 said he hoped the plan, which includes the rotation of the 30 doctors still in eastern Aleppo, could be carried out “in the next few days”.

The siege and intense bombardment of east Aleppo, aggravated by renewed, frequent airstrikes 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 people were 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.”

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 UN has said.

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 ‘four towns’ this weekend and it is urgent.”