Aleppo civilians killed in 'complete meltdown of humanity': UN

Dozens of civilians were killed by Syrian forces in "a complete meltdown of humanity" during the final battle for Aleppo, the U.N. said Tuesday amid separate reports that women and children were burned alive while some families chose suicide over surrender.

The U.N. human rights office said it received reports of pro-government forces killing at least 82 people as they tightened their grip on the shrinking rebel districts in the east of the city.

Related: Children affected by conflict in Aleppo

Rupert Colville, spokesman of the U.N. human rights office, said he feared retribution against thousands of civilians holed up in a "hellish corner" smaller than one square mile.

Eleven women and 13 children were among those killed in four different neighborhoods late Monday, Colville told a news briefing, adding that there could be "many more".

"The reports we had are of people being shot in the street trying to flee and shot in their homes."

Jens Laerke, U.N. humanitarian spokesman said that it looked like "a complete meltdown of humanity in Aleppo".

U.N. agency UNICEF said "many children, possibly more than 100, unaccompanied or separated from their families, are trapped in a building, under heavy attack in east Aleppo," citing reports form a doctor on the scene.

Arab media reported that scores of civilians were burned alive by regime forces, although this was not confirmed by observers at the Aleppo Media Center or the U.K.-based Syria Observatory for Human Rights.

Charles Lister, a Syria expert and senior fellow at the Middle East Institute, said there were "truly shocking stories from Aleppo including husbands and wives taking each other's lives in family suicides" and that hundreds may have died during Monday's fighting.

Trapped activists posted farewell messages on social media, saying they were "waiting for death or captivity" while others fled to government-held areas. As the frontlines quickly shifted, thousands carried what possessions they could, some pushing relatives in wheelchairs, before a heavy overnight rainstorm.

Celebrations on the government-held side of the divided city lasted long into the night, with a Reuters witness describing bullets dropping "like rainfall" as fighters shot into the air in triumph.

The Red Cross on Tuesday warned of a "deepening humanitarian catastrophe" and said further loss of life "can be averted only if the basic rules of warfare - and of humanity - are applied."

Related: Aleppo before the war

It said in a statement: " Thousands of civilians' lives are in danger .... 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."

U.N. Secretary-General Ban Ki-Moon said he was "alarmed over reports of atrocities against a large number of civilians," but stressed the U.N. was "not able to independently verify these reports."

Ismail Alabdullah, a volunteer with the White Helmets, spoke to NBC News from Aleppo. He said bombing was so fierce the volunteer group were unable to carry out rescue missions.

In an emotional Skype interview he said: "All the world let us down, even they can't protect our volunteers or protect the civilians, so we can't carry out our missions".

"We're not scared of bombing anymore, all we're scared of is Assad's forces more than ever. Me and everyone, even the white helmets."

In the rebel area, photographer Ameen Al-Halabi posted what he described as a "last letter" on Facebook.

"I'm waiting for death or captivity by the Assad regime," he wrote, saying that "to die victorious on earth dust is better than despair ... please forgive me and pray for forgiveness and remember me well."

Twitter user 'Mr.Alhamdo,' who says he is an Aleppo resident, posted a video update saying farewell to his followers.

"I don't believe any more in the U.N., don't believe any more in the international community," he said. "Russia doesn't want us to go out alive, they wants us dead. Assad is the same.

"Yesterday there many celebrations on the other part of Aleppo, they were celebrating on our bodies. It's okay, this is life but at least we know that we were a free people, we wanted freedom. This world doesn't like freedom it seems. Don't believe that you are free people in your countries any more. "

Another user, Monther Etaky, posted: "I would like to thank all the humans whose stood ... with our [cause] ... I will never forget you if we pass to the other life."

None of the accounts could be independently verified by NBC News. Syria's military says the claims of atrocities are "a desperate attempt" by rebel forces to gain international sympathy.

Turkey said it was negotiating with Russia to open a corridor to evacuate rebel fighters and civilians but that there has been "no agreement on this issue yet," a Turkish official told Reuters.

Russia said it had helped more than 110,000 civilians, including 44,367 children, from rebel-held Aleppo since the Syrian operation to retake it began at the end of last month.

France called for an investigation into the alleged atrocities, warning Russia that it risked being complicit in acts of "vengeance and terror."

"The backers of the regime, starting with Russia, cannot allow this logic of vengeance and terror without taking the risk of being complicit," foreign minister Jean-Marc Ayrault said, adding that the international community must not "let these crimes remain unpunished."

Earlier, U.N. humanitarian adviser on Syria Jan Egeland said Syria and Russia were "accountable for any and all atrocities that the victorious militias in Aleppo are now committing."