UN Secretary-General Ban Ki-moon has voiced alarm “over reports of atrocities against a large number of civilians” in Aleppo.
Mr Ban urged all the sides, particularly Syria’s government and its allies, to protect civilians.
UN humanitarian adviser Jan Egeland earlier said Syria and Russia were “accountable” for any atrocities committed by pro-government militias.
Rebels inside Aleppo are on the brink of defeat after Syrian army gains.
In a statement, Mr Ban’s spokesman Stephane Dujarric said “the secretary-general is alarmed over reports of atrocities against a large number of civilians, including women and children, in recent hours in Aleppo”.
“While stressing that the United Nations is not able to independently verify these reports, the secretary-general is conveying his grave concern to the relevant parties.
“He has instructed his special envoy for Syria to follow up urgently with the parties concerned,” the statement added.
A US official with knowledge of efforts to secure safe passage for people trapped in the city told the BBC that some
“50,000 people remain under bombardment” in Aleppo with the Russians rejecting the idea of pauses in the bombing to allow people to leave or allow injured children to be evacuated.
So far there have been no public comments on the issue by Syria’s government or its ally Russia.
Mr Egeland earlier tweeted that the governments of Syria and Russia were “accountable for any and all atrocities that the victorious militias in Aleppo are now committing”.
