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.
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.
Witnesses have also told UN that relatives have been unable to retrieve the bodies of those shot dead in the street due to the heavy bombardment. Syrian troops are on the verge of recapturing all of Aleppo, which the rebels fighting against President Bashar al-Assad first claimed in 2012.