Syrian rebels agree to aid deliveries to east Aleppo, UN says, but no official confirmation yet from Russia or Syria.

Syrian rebels in besieged east Aleppo have agreed to a plan for aid deliveries and medical evacuations, according to UN officials, but the global body is awaiting a “green light” from Russia and the Syrian government before it can begin life-saving operations.

Jan Egeland, the head of a UN-backed humanitarian task force for Syria, said on Thursday that his team had received written approval from Syrian rebels and “verbal support” from Russia for the United Nations plan reached earlier this month.

The four-point plan involves the supply of medical and food aid, the evacuation of 200 wounded or disabled civilians, and medical staff to rotate in and out of besieged areas.

“We have written approval in principle by the armed opposition groups of east Aleppo,” Egeland told reporters in Geneva, specifying that he was referring to rebels with whom the UN is in contact – which does not include Jabhat Fateh al-Sham, the group formerly known as al-Nusra Front.

“We hope to have a full green light from both the Russian side and Syrian government side.”

Asked about whether there was a “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.”

READ MORE: Fears grow in east Aleppo as government forces close in

Once Syria’s economic powerhouse, Aleppo has been devastated by fighting since the rebels seized the east of the city in 2012, turning its historic heart into a battlefield.

More than 250,000 civilians are still trapped in the divided city and are at risk of mass starvation as the winter sets in. Lorries are waiting in Turkey and west Aleppo to bring in aid, but the UN needs 72 hours to prepare the “big, complex and dangerous operation”, Egeland added.

Dozens of civilians tried to flee east Aleppo on Wednesday but were forced to retreat by gunfire, local sources told Al Jazeera, as the Syrian army – backed by allied forces from Iran and the Lebanese group Hezbollah – continued its assault on the divided city.

“Two or three families paid smugglers and were attempting to cross when clashes and gunfire broke out and forced them back,” Aleppo-based journalist Zouhir al-Shimale told Al Jazeera.

INFOGRAPHIC: Aleppo – who controls what?

Earlier this week, Stephen O’Brien, the UN humanitarian chief, said conditions in the rebel-held areas had gone from “terrible to terrifying” and were “barely survivable by human beings”.

“Let me be clear, we are not just seeing a resumption of violence in Aleppo. This is not business as usual. What has been unleashed on civilians this past week is yet another low in an unrelenting, inhuman onslaught, and it is as heartbreaking as it is not inevitable,” O’Brien told the UN Security Council on Monday.

“These parties to the conflict are – all of them – choosing to do this. It is civilians who pay the price.”

Rebel forces have steadily lost ground since Russia, a key backer of President Bashar al-Assad, intervened to bolster his government last year.

Recapturing eastern Aleppo would give Assad’s government perhaps its most important victory yet in the conflict, which has killed an estimated 400,000 people since it began in March 2011.