Civilians and opposition fighters will start evacuating east Aleppo "within hours" under a deal with Syria's regime, a rebel official said Tuesday, as global outrage mounted over reports of atrocities including summary executions.
Yasser al-Youssef from the political office of the key Nurredin al-Zinki group told AFP the deal with President Bashar al-Assad's regime was being "sponsored" by Russia and Turkey.
"An agreement has been reached," Youssef said. "The first stage will be the evacuation of civilians and wounded, within hours, and afterwards fighters will leave with their light weapons."
Those leaving will be allowed to travel to other rebel-held territory in the west of Aleppo province or neighbouring Idlib province, he said.
A source in the powerful Ahrar al-Sham rebel group confirmed the deal and its details.
There was no immediate confirmation from the regime, Ankara or Moscow.
The United Nations and aid agencies have been pleading for a ceasefire to allow for the evacuation of tens of thousands of civilians trapped in the last pocket of rebel territory in Aleppo.
After weeks of heavy fighting, forces loyal to Assad were in the last hours of a push to take full control of the city, dealing the biggest blow to Syria's rebellion in more than five years of civil war.
But as the long battle reached its final stages, the United Nations said it had received reports of at least 82 civilians, including 11 women and 13 children, being executed by pro-government forces.
Following a request from Britain and France, the UN Security Council was to hold an emergency meeting at 1700 GMT to address what the French envoy called "the worst humanitarian tragedy of the 21st century unfolding before our eyes".
- 'Last hellish corner' -
Speaking to reporters in Geneva, UN rights office spokesman Rupert Colville said it had received credible reports of the civilian executions in recent days.
Pro-government fighters had in some cases entered homes and killed those inside, and in others "caught and killed on the spot" fleeing civilians, he said.
The UN was "filled with the deepest foreboding for those who remain in this last hellish corner of opposition-held eastern Aleppo", Colville said.
Residents in remaining rebel-held territory told AFP they had no hope left.
"Our fate is sealed. Why would we hide, it won't do us any good. We will either die or be captured," said Ibrahim Abu al-Leith, a spokesman for the White Helmets rescue service.
Other witnesses described scenes of carnage in rebel areas, with bodies lying amid the rubble of city streets, as desperate residents sat on pavements with nowhere to shelter.
French Foreign Minister Jean-Marc Ayrault said there were worrying reports of "cold-blooded killings of entire families", "summary executions, including of women and children" and "people burned alive in their homes".
"Such atrocities are unconscionable. Supporters of the regime, starting with Russia, cannot let this happen."
Britain repeated its call for Assad to step aside, citing his "barbaric cruelty", and French Prime Minister Bernard Cazeneuve said "massacres" in Aleppo could amount to war crimes and crimes against humanity.
UN chief Ban Ki-moon had late Monday also expressed alarm "over reports of atrocities against a large number of civilians, including women and children".
- 'Last chance to save lives' -
Syria's army has taken more than 90 percent of the territory once held by rebels in east Aleppo, after launching an all-out offensive last month to seize control of the entire city.
Aleppo, a cultural and economic hub in northern Syria second only to Damascus in importance, had been split between a rebel-controlled east and government-held west since 2012.
Recapturing all of Aleppo will be a huge victory for Assad and leave his regime in control of all five of Syria's main cities.
As of early Tuesday, rebels were reported to be confined to just a handful of neighbourhoods, including Mashhad and part of Sukkari.
It was unclear how many civilians remained in rebel territory, after an estimated 130,000 fled to other parts of Aleppo during the government advance.
Jan Egeland, head of the UN-backed humanitarian taskforce for Syria, told AFP that thousands of civilians were in need of evacuation and safe passage, including hundreds of wounded.
"We need a pause in the fighting but we also need to get the people inside to help organise an evacuation," he said.
The International Committee of the Red Cross said there was only a "last chance to save lives" in Aleppo.
"As the battle reaches new peaks and the area is plunged into chaos, thousands with no part in the violence have literally nowhere safe to run," it said.
- Trapped children -
The UN's children agency UNICEF said it had "alarming reports" from a doctor in Aleppo of more than 100 children, unaccompanied or separated from their families, trapped in a building under heavy attack.
UNICEF regional director Geert Cappelaere said in a statement the agency was also "deeply concerned by unverified reports of extra judicial killings of civilians including children".
The government assault has been backed by heavy artillery fire and air strikes, with at least 463 civilians, including 62 children, killed in east Aleppo since mid-November, according to the Syrian Observatory for Human Rights.
Another 130 people including 40 children have been killed in western districts by rebel rocket fire, the Britain-based monitoring group says.
More than 310,000 people have been killed since the Syrian conflict began in March 2011, and over half the population has been displaced, with millions becoming refugees.
