The last remaining food rations are being distributed in besieged rebel-held eastern districts of the Syrian city of Aleppo, the UN has said.
Humanitarian adviser Jan Egeland warned that without a resupply there would be no food left to hand out next week to the 275,000 people living there.
Mr Egeland ruled out airdrops of food, explaining that they were not possible in densely-populated urban areas.
Government forces launched a major assault on eastern Aleppo in September.
Since then, troops have pushed into several outlying areas with the help of Iranian-backed Shia militias and Russian air strikes.
Rebels launched a counter-attack in an attempt to break the siege in late October. But their progress slowed after early gains.
The UN says weeks of air strikes and shelling have killed more than 700 civilians in the east, while rocket-fire has left scores dead in the government-controlled west.
Humanitarian agencies have been unable to get into rebel-held Aleppo since the government siege resumed in September, and the last time significant aid supplies were delivered was in July.
On Thursday, Mr Egeland told journalists in Geneva that the UN’s humanitarian taskforce had received a report that the last remaining food rations were being handed out.
“[Food] prices are now skyrocketing on the market. We have reports of desperation for families and within communities,” he said.
He warned that the “terrible” situation in eastern Aleppo might be about to get “much, much worse”, with Russia saying it would soon resume air strikes after a three-week pause intended to allow civilians and rebels to be evacuated – an offer very few people took up.
Mr Egeland revealed that the UN had presented all the warring parties and their backers last week with a four-point initiative for getting aid to eastern Aleppo.
The plan would involve medical evacuations for about 300 patients in urgent need, delivery of food and medical supplies, and permission for medical personnel to enter the city.
Mr Egeland said he was confident that access would be granted because “the consequences of no help and no supplies will be so catastrophic I cannot even see that scenario”.
But he admitted that he had never seen such “politicisation and manipulation” of aid as was taking place in Syria, adding that the constant preconditions being imposed on aid operations were making successful deliveries of supplies impossible.
Reacting to the result of the US presidential election, Mr Egeland said the UN needed “uninterrupted, full engagement” from the US over Syria, pointing out that the only progress made at all had taken place when the US and Russia were involved and using their influence.
The Obama administration has backed the opposition to Syrian President Bashar al-Assad since the uprising against him erupted in 2011 and also condemned Russia’s air campaign. But President-elect Donald Trump has said he wants to mend ties with Moscow and is against regime change in the Middle East.