Syrian pro-government forces in
eastern Aleppo have been killing people, including women and children,
on the spot in their homes and on the street, the United Nations says.
The UN's human rights office said streets were full of bodies.Meanwhile, the UN children's agency cited a doctor as saying a building housing as many as 100 unaccompanied children was under heavy attack.
Rebels, who have held east Aleppo for four years, are on the brink of defeat.
Thousands of people are reportedly trapped in the last remaining neighbourhoods still in rebel hands, facing intense bombardment as pro-government troops advance.
The Syrian government's ally Russia, which has rejected calls for a humanitarian truce, earlier said any atrocities were "actually being committed by terrorist groups", meaning rebel forces.
The UN Security Council will discuss the situation in Aleppo later on Tuesday.
What's happening in rebel areas?
"We're filled with the deepest foreboding for those who remain in this last hellish corner" of eastern Aleppo, UN human rights office spokesman Rupert Colville told a news conference.He said that 82 civilians had reportedly been killed by pro-government forces, of whom 11 were women and 13 children, adding that the death toll could be much higher.
"Yesterday evening, we received further deeply disturbing reports that numerous bodies were lying on the streets," Mr Colville added, while admitting it was hard to verify the reports.
"The residents were unable to retrieve them due to the intense bombardment and their fear of being shot on sight."
Meanwhile, Unicef quoted a doctor in the city as saying: "Many children, possibly more than 100, unaccompanied or separated from their families, are trapped in a building, under heavy attack in east Aleppo".
Ibrahim abu-Laith, a spokesman for the White Helmets volunteer rescue group, said 90% of their equipment was out of operation and only one medical point was still working in the besieged areas. There was no first aid equipment left, he added.
He said volunteers were using their hands to pull people out of rubble, but some 70 people were stuck and could not be extracted.
It is hard to know exactly how many people are in the besieged areas, although one US official with knowledge of efforts to secure safe passage for people in the city told the BBC that there were around 50,000 people.
Other local sources say there could be as many as 100,000, with people arriving from areas recently taken by the government.