The United Nations warned that it had received reports of dozens of extrajudicial executions by Syrian government forces in eastern Aleppo, amid appeals for protection of civilians.

Government forces have reclaimed the majority of the rebel enclave in eastern Aleppo and are poised to recapture the few areas still in rebel possession.

At least 82 civilians have reportedly been killed during the offensive, UN human rights spokesperson Rupert Colville said, though he cautioned that reports were very hard to verify thanks to an "extremely fluid" situation.

The British-based Syrian Observatory for Human Rights said that around 60 people were killed in the neighbourhood of al-Faradous by either regime fire or shelling after government troops took the area from rebels on Monday.

The Observatory could not confirm reports of extrajudicial executions.

The head of the UN's humanitarian task force for Syria urged the Syrian government and its ally Russia to agree to a ceasefire so that injured people could be given treatment.

"Our renewed urgent appeal to Russia and Damascus: ceasefire today to let us evacuate wounded and other vulnerable groups from Aleppo's rubble," Jan Egeland posted on his Twitter account.

Egeland also said that Moscow and Damascus "are accountable for any and all atrocities that the victorious (pro-government) militias in Aleppo are now committing."

The International Committee of the Red Cross (ICRC) said that thousands of civilians were at risk as front lines closed around them, warning that "this may be the last chance to save lives."

It said it had been in touch with all sides to find a humanitarian solution for more than a week, without success.

"We stand ready to oversee the implementation of any mutual agreement that puts civilians first. We cannot urge this strongly enough: this must happen now," said Marianne Gasser, the head of the international humanitarian organisation's delegation in Syria.

Limited fighting and airstrikes were reported in the enclave today as heavy rain apparently hampered military operations.

Government forces made rapid advances yesterday, hemming in rebels and remaining civilians in an area less than 3 percent the size of the original enclave.

Opposition leaders and people inside eastern Aleppo fear deadly reprisals by the Syrian government. Unarmed activists and medics in particular are concerned, with several telling dpa that they expect arrest and torture if they are caught.

Activists and civil defence workers were "at risk of grave violations including detention, torture and killing," Colville warned.

He noted reports from Aleppo that some civilians trying to leave the area had been caught and killed on the spot and others arrested, as well as reports of pro-government forces breaking into houses and killing the inhabitants.

"We hope, profoundly, that these reports are wrong, or exaggerated, as the situation is extremely fluid and it is very challenging to verify reports.

However, they have been corroborated by multiple reliable sources," Colville added.

International monitoring was needed to "alleviate 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," he said.

"The news out of eastern Aleppo is catastrophic...people have to be saved or else they will be massacred by the regime and their allies when they reach them," Samir al Nashar, a Syrian opposition figure originally from Aleppo, told dpa.

The Observatory said that nearly 600 civilians have been killed in the last month since regime forces launched the offensive that has seen them seize almost all of the rebel enclave in eastern Aleppo.

Some 463 civilians were killed in airstrikes and shelling in the east, and another 130 were killed in rebel shelling in the government-held western sector.

Rebel defences in eastern Aleppo collapsed in late November after a five-month siege by government forces.

- dpa