Spokesman Stephane Dujarric said rebel-held parts of Aleppo had received no aid since the beginning of July, and food was becoming scarce.
Supplies provided by the World Food Programme ran out on 13 November and aid from other agencies is dwindling, “effectively leaving eastern Aleppo city without food,” he said.
The UN said it is “extremely concerned” about the estimated 275,000 civilians trapped in “horrific conditions”.
“We urge all parties to the conflict to put an end to the indiscriminate bombing and shelling and to protect civilians and civilian infrastructures and to enable life-saving humanitarian assistance, as required under international humanitarian law,” Ms Dujarric said.
Faced with devastating bombing and street fighting, thousands are fleeing to safety after more than four months of siege.
