Fuel supplies could run out within days in a rebel-held part of the Syrian city of Aleppo, the UN special envoy for Syria has warned.
Fighting has escalated between Syria's army and rebels in eastern Aleppo, where 250,000 people live.
Staffan de Mistura said food and water shortages made the situation there even more serious than previously.
He said a ceasefire deal at US-Russian talks in Geneva could make a major difference to aid efforts across Syria.
The US has said the talks are "making progress" but Russian Foreign Minister Sergei Lavrov has said Washington is delaying a deal.
On Sunday, Syrian government forces were reported to have recaptured parts of Aleppo city which were lost to rebels last month, placing rebel-held districts in the city's east once again under siege.
A monitoring group said government troops had recaptured two military academy sites in the Ramouseh district, in the south of the city, and severed a recently established rebel supply line.
'Apex of horror'
"There is a growing concern about eastern Aleppo: the issue about food, the issue about the possibility that within perhaps the next few days it will turn out to be dark because there is no fuel, problems of water," Mr de Mistura said on Friday.
UN humanitarian chief Stephen O'Brien told the BBC conditions in Aleppo had become appalling: "Eastern Aleppo is at the apex of horror, where anyone of us if we were there would find life barely possible, let alone tolerable."
Mr O'Brien said civilians were trapped in ruined buildings, subjected to daily bombing and shelling.
He described children in the city as a "lost" generation, who had no access to school.
Over the summer the UN's ability to deliver aid across Syria was greatly reduced because of the increased fighting, the UN said.
Little aid was delivered in July, less in August, and none at all so far in September, it added.
Another decade of bloodshed? Analysis by Jeremy Bowen, BBC Middle East editor, Damascus
A ceasefire deal that the Russians and the Americans arranged back in February collapsed quickly in the north but it is still more or less holding, or at least influencing matters, around Damascus.
That does not mean that the war is anywhere near over. A Syrian general at the ministry of defence told me they were well aware that the war in Lebanon a generation ago had lasted 16 years.
This one, he said, was much more complicated so there could be at least another 10 years of bloodshed. The collective tragedy is that so many foreign countries have intervened in the war that it has become much harder to stop.
Syria's war is a big part of the historic change sweeping through the Middle East, and it is tied into other conflicts. Power is shifting, with global repercussions. More tragedies lie ahead.
The UN warnings come as US Secretary of State John Kerry and Mr Lavrov continue their talks about a ceasefire in Syria.
A US state department official said the two sides were "making progress... towards advancing proposals that would lead to a nationwide cessation of hostilities in Syria, as well as sustained and unimpeded access to humanitarian assistance for communities most in need".
However, the official stressed that "technical details" needed to be "worked out".
Mr Kerry and Mr Lavrov have been meeting on and off for several weeks now and the negotiations are clearly proving complicated, the BBC's Imogen Foulkes in Geneva reports.
The US and Russia support opposite sides in the conflict that began in 2011: Washington backs a coalition of rebel groups it describes as moderate, while Moscow is seen as a key ally of Syrian President Bashar al-Assad.