Hundreds of men appear to have gone missing after crossing from rebel-held areas of Aleppo into government territory, UN officials say.
Forces led by Syria's government have seized at least 85% of eastern parts of the city from rebels in recent weeks.
Tens of thousands of civilians have fled those districts. Up to 10,500 left during a humanitarian pause on Thursday alone, Russian officials say.
Rebels were also reportedly stopping people from leaving, the UN said.
What are the allegations?
Rupert Colville, the spokesman for the UN High Commissioner on Human Rights, saidup to 100,000 people were trapped in "ever-shrinking" areas of eastern Aleppo.
Reports differ on how many people remain and how many have fled eastern Aleppo, but Mr Colville said the UN had gathered evidence that "hundreds" of men may have disappeared after leaving for government-held areas.
"Given the terrible record of arbitrary detention, torture and enforced disappearances by the Syrian government, we are of course deeply concerned about the fate of these individuals," he said.
Reports cited by the UN say men aged between 30 and 50 were separated from their families. Other displaced people reported being taken in for questioning, and having their identity cards confiscated.
Mr Colville also said rebel groups could be committing a war crime by preventing people from fleeing to safety, and "using civilians as pawns".