GENEVA (REUTERS) - Hundreds of men from eastern Aleppo have gone missing after leaving rebel-held areas, the United Nations' human rights office said on Friday (Dec 9), voicing deep concern that government forces could be mistreating them.

UN human rights spokesman Rupert Colville also said there were reports that two rebel militias - Jabhat Fatah al-Sham, formerly known as the Nusra Front, and the Abu Amara Battalion - had during the last two weeks abducted and killed an unknown number of civilians in the city who had asked armed groups to leave their neighbourhoods to save the lives of civilians.

Syrian government forces pressed on with their offensive in Aleppo on Thursday night and into Friday with ground fighting and air strikes, Reuters witnesses, rebels and a monitoring group said, part of a push to retake all of the city's besieged rebel-held east.

"As pro-government forces have advanced from the north into eastern Aleppo, there have been allegations of reprisals against civilians who are perceived to have supported armed opposition groups, as well as reports that men were being separated from women and children," Colville told a news briefing in Geneva.

"We have received very worrying allegations that hundreds of men have gone missing after crossing into government-controlled areas.

The families of the men, who are mainly between 30 and 50 years old, had not heard from them since they fled a week to 10 days ago, he said, adding that it was not clear whether they were civilians.

"Given the terrible record of arbitrary detention, torture and enforced disappearances, we are of course deeply concerned about the fate of these individuals," Colville said. "One has to ring some alarm bells."

"It could mean that some have been killed, it could mean they have been arbitrarily detained and taken somewhere, we just don't know."

A senior official of the International Committee of the Red Cross (ICRC) told Reuters on Dec 1 that the agency was in talks with the government about gaining access to people fleeing rebel-held eastern Aleppo who were being screened or detained.

Colville said that if rebels were proven to have prevented civilians fleeing to safety, this could amount to a war crime.

"Civilians are caught between warring parties that appear to be operating in flagrant violation of international humanitarian law."

He said tens of thousands had fled the shrinking opposition-held areas of the city, but that at least 100,000 civilians were believed to remain.