According to BBC, 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 neighborhoods still in rebel hands, facing intense bombardment as pro-government troops advance. BBC reports.
Both the UN and the International Committee of the Red Cross (ICRC) have appealed to Syria and its ally Russia to end the bombardment and allow civilians a safe passage out.