Syrian pro-government forces have been entering homes in eastern Aleppo and killing those inside, including women and children, the UN says.The UN's human rights office said it had reliable evidence that in four areas 82 civilians were shot on sight.
"We're filled with the deepest foreboding for those who remain in this last hellish corner" of eastern Aleppo, said spokesman Rupert Colville.
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.More than 100 unaccompanied children are trapped in a building which is under heavy attack in eastern Aleppo, according to UNICEF.
The UN children's agency's regional director Geert Cappelaere said: "According to alarming reports from a doctor in the city, many children, possibly more than 100, unaccompanied or separated from their families, are trapped in a building, under heavy attack in east Aleppo."
Russia, which has rejected calls for a humanitarian truce, earlier said any atrocities were "actually being committed by terrorist groups", meaning rebel forces.
At a news conference in Geneva, Mr Colville gave details of atrocities reportedly carried out by pro-government troops - admitting it was impossible to verify their accuracy.
He said that of the 82 civilians reportedly shot, 11 were women and 13 were children.
"Yesterday evening, we received further deeply disturbing reports that numerous bodies were lying on the streets," Mr Colville added.
"The residents were unable to retrieve them due to the intense bombardment and their fear of being shot on sight."
It is hard to know exactly how many people are trapped 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.
Some residents have sent out messages saying they are crowded into abandoned apartments and rainy streets, unable to take shelter from the bombing, the New York Times reports.
Many are said to be fearful about what will happen to them after the city falls, particularly since the allegations of summary killings in areas that had already fallen became known.According to the AFP news agency, the rebels have control of just a handful of neighbourhoods, including Sukkari and Mashhad.
The Syrian army's Lt Gen Zaid al-Saleh said on Monday that the battle "should end quickly", telling the rebels they "either have to surrender or die".
The British-based monitoring group, the Syrian Observatory of Human Rights (SOHR), has also said the battle for Aleppo had reached its end, with "just a matter of a small period of time" before "a total collapse".