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.
Russia, which has rejected calls for a humanitarian truce, earlier said any atrocities were “actually being committed by terrorist groups”, meaning rebel forces.
What’s happening in rebel areas?
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.
The Russian Centre for Reconciliation of the Opposing Sides says it has helped 7,796 civilians leave rebel-held areas in the past 24 hours.
How close are rebel-held areas to falling?
It is not entirely clear, but Russia’s military, allied to the Syrian government forces, says 98% of the city is now back in government hands.
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”.
What is the world saying about it?
UN Secretary-General Ban Ki-moon has said he is alarmed by the reports of atrocities and has instructed his special envoy to Syria to “follow up urgently with the parties concerned”.
The ICRC has said people have “literally nowhere safe to run”, and warned that the basic rules of war and humanity must be respected to avert a humanitarian crisis.
The UN’s humanitarian adviser on Syria, Jan Egeland, earlier spoke of “massacres of unarmed civilians, of young men, of women, children, health workers”, and held Russia and Syria responsible.
In response, Kremlin spokesman Dmitry Peskov said Mr Egeland was not in full possession of the facts on the ground. “If he did, he would be paying attention to the atrocities that are actually being committed by terrorist groups,” he said.
For much of the past four years, Aleppo has been divided roughly in two, with the government controlling the western half and rebels the east.
Syrian troops finally broke the deadlock with the help of Iranian-backed militias and Russian air strikes, reinstating a siege on the east in early September and launching an all-out assault weeks later.
Analysts say the fall of Aleppo would be a big blow to the opposition, as it would leave the government in control of Syria’s four largest cities.