The U.N. 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 of the U.N. human rights office says the reports recount pro-government forces entering homes and killing some civilians "on the spot" in the former rebel enclave.

Colville spoke to reporters in Geneva on Tuesday.

He says 11 women and 13 children were among those reportedly killed in four neighborhoods of the increasingly-shrinking rebel enclave in the city of Aleppo.

Colville said he feared retribution against thousands of civilians believed to be holed up in a "hellish corner" of less than a square kilometer of opposition-held areas. Its capture was imminent, he said.

"The reports we had are of people being shot in the street trying to flee and shot in their homes," Colville said. "There could be many more."

"The only way to alleviate the deep foreboding and suspicion that massive crimes may be under way both within Aleppo, and in relation to some of those who fled or were captured, whether fighters or civilians, is for there to be monitoring by external bodies, such as the UN," Colville said.

Colville says the reports came in late the previous evening and that he doesn't know exactly when the killings took place.