The UN human rights office says it has received reports of Syria's pro-government forces killing at least 82 civilians as they entered the last remaining strongholds of the rebels in eastern Aleppo.
Spokesman Rupert Colville said the reports recount pro-government forces entering homes and killing some civilians "on the spot" in the former rebel enclave.
Mr Colville spoke to reporters in Geneva on Tuesday.
He said 11 women and 13 children were among those reportedly killed in four neighbourhoods of the increasingly-shrinking rebel enclave in the city of Aleppo.
Mr Colville said the reports came in late the previous evening and that he does not know exactly when the killings took place.
Meanwhile, the UN humanitarian aid agency says its partners in Syria have recorded about 37,000 people who have fled eastern Aleppo, but that thousands of others have not been counted.
Spokesman Jens Laerke of OCHA condemned the "complete meltdown of humanity" in Aleppo, and said that untold thousands are believed to remain behind in shrinking areas held by rebel fighters.
Mr Laerke said in Geneva his office understands that "many civilians have not fled, and may be hiding whether they can".
He says the UN and its partners, including the Syrian Arab Red Crescent, are actively working to get "blanket access" across the city - not just in western Aleppo that has long been controlled by government forces and their allies.