The UN’s human rights office said streets were full of bodies, the BBC reports.
Meanwhile, the UN children’s agency (UNICEF) 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 neighborhoods 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.