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 75% of eastern parts of the city from rebels in recent weeks.
Tens of thousands of civilians have fled those districts, with more than 8,000 leaving during a humanitarian pause on Thursday.
Rebels were also reportedly stopping people from leaving, the UN said.
At a briefing in Geneva, Rupert Colville, the spokesman for the UN High Commissioner on Human Rights, said up 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 had disappeared after leaving for government-held areas.
"Given the terrible record of arbitrary detention, torture and disappearances, we are of course deeply concerned," he said, referring to the Syrian government.
Some reports cited by the UN say men were separated from families and have now disappeared. Other displaced people reported being taken in for questioning, and having their identity cards confiscated.
Mr Colville also said there had been reports that rebel groups were preventing people from fleeing to safety, and were "using civilians as pawns".
On Thursday, Russia, an ally of Syrian President Bashar al-Assad, said fighting was suspended to allow civilians to leave.
By Friday morning, more than 8,000 people - including 3,000 children - had left, Russia's military said. But fighting would continue, Russian Foreign Minister Sergei Lavrov said, "until the bandits leave east Aleppo".
The UK-based Syrian Observatory for Human Rights, a network of activists monitoring the violence, said ground forces continued their offensive, and that rocket attacks and air raids were launched overnight.
In other developments:
- The civilian rescue group known as the White Helmets said 46 civilians were killed and another 230 injured on Thursday in east Aleppo
- Three barrel bombs carrying chlorine gas were dropped, it added
- The past two days have seen some of the heaviest civilian casualties in the past fortnight in rebel-held areas.
After several previous attempts to flee the besieged area of Salhine, one man, Abdel Hamid, managed to leave with his wife and 10 children on Thursday.
"Most of the people around me were saying 'in any case we will die, so let's leave together'. That encouraged me and we left," he told the AFP news agency.
"I left my house behind... but I have secured my children's right to live. With each step I took I felt like I was getting closer to life itself."
Envoy to meet Trump team
Mr Lavrov confirmed that Russian and US military experts would meet in Geneva on Saturday to discuss ways of ending the violence in Aleppo.
Mr Lavrov said the talks would focus on plans to evacuate rebel fighters from eastern Aleppo, but the US state department said the subject had yet to be agreed. Rebels in the city have said they intend to fight on.
The UN's envoy for Syria, Staffan de Mistura, told reporters in New York on Thursday that he was planning to meet members of US President-elect Donald Trump's team, though he did not say when.
Aleppo was once Syria's largest city and its commercial and industrial hub before the uprising against President Bashar al-Assad began in 2011.
It has been divided in roughly two since mid-2012. But in the past year, Syrian troops have broken the deadlock with the help of Iranian-backed militias and Russian air strikes, reinstating a siege in early September.