The Syrian army and its allies have taken full control over all the Aleppo districts abandoned by rebels during their retreat in the city, a Syrian military source said on Tuesday.
Rupert Colville, spokesman for the UN human rights office, said he feared retribution against thousands of civilians believed to be holed up in a "hellish corner" of less than a square kilometre of opposition-held areas. Its capture was imminent, he said.
"In all, as of yesterday (Monday) evening we have received reports of pro-government forces killing least 82 civilians, including 11 women and 13 children, in 4 different neighbourhoods - Bustan al-Qasr, al-Fardous, Al-Kalasah and al-Saliheen," Colville told a news briefing, naming the Iraqi armed group Harakat al-Nujaba as reportedly involved in the killings.
"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.
The UN's children's agency warned that dozens of unaccompanied children were trapped in a building under fire in eastern Aleppo and called for their immediate evacuation from the rebel enclave.
UNICEF said in a statement on Tuesday that there could be more than 100 children trapped in the building, according to reports from an unnamed doctor in the city.
The organization's regional director, Geert Cappelaere, says it's "time for the world to stand up for the children of Aleppo and bring their living nightmare to an end."
Thousands of people fled the front lines of fighting in Aleppo on Tuesday with the onset of the Syrian advance, and a senior Turkish official said Russia and Turkey would meet to try to set up an evacuation corridor.
But international efforts to seek a negotiated settlement to end fighting in Aleppo have shown no signs of a breakthrough, with Russia and the United States exchanging recriminations over hitches in ceasefire talks.
France on Tuesday called on the United Nations to use all its mechanisms to determine what was happening in Aleppo, warning Russia that it risked being complicit in acts of "vengeance and terror" taking place in the Syrian city.
French President Francois Hollande pressed for Russia to facilitate humanitarian aid to civilians trapped in rebel-held parts of the city, saying the Aleppo "humanitarian situation ... is unacceptable."
The French leader said 120,000 people were being "held hostage, there is no other word for it—who are victims of bombing, who are victims of repression" in Aleppo and that everything must be done to allow the population's evacuation.
Jens Laerke, UN humanitarian spokesman said that it looked like "a complete meltdown of humanity in Aleppo".