Aid workers have said the streets are "full with dead bodies" as 100,000 civilians remain trapped
Residents and aid workers trapped in Aleppo are describing desperate conditions and a "complete meltdown of humanity" as the Syrian army is about to regain full control of the city.
President Bashar al Assad's forces, backed by Russia, have recaptured large swathes of the city in recent days and, according to state TV, it now holds 98% of neighbourhoods previously occupied by rebels.
The UN rights office cited reports of Syria's pro-government forces killing at least 82 civilians in rebel-held eastern Aleppo.
A UN spokesman spoke of a "complete meltdown of humanity" in the city.
A Syrian military source said government forces and their allies are in the "last moments before declaring victory", while the head of the government's Aleppo security committee warned rebels they "either have to surrender or die".
Lieutenant General Zaid al-Saleh added: "The battle in eastern Aleppo should end quickly. They (rebels) don't have much time."
Residents sent texts, tweets and videos to make dramatic appeals for help or say goodbye, as they said they feared they would die or be arrested by government troops.
Abdulkafi Alhamdo, a teacher turned activist, posted a "last call" video on Monday, calling the crisis in the city "the most horrible massacre … in our history."
The White Helmets civil defence organisation and three other aid groups asked the international community to arrange safe passage for 100,000 civilians across a 4km stretch of government-held territory.
"If we stay, we fear for our lives. The women may be taken to camps, the men disappeared and anyone who is known to have supported civilians will face detention or execution," they said in a statement.
In this moment we serve as a reminder that humanity will always prevail and defy dictatorship. We will not kneel. #saveAleppo— The White Helmets (@SyriaCivilDef) December 13, 2016
There is no total number of casualties in Besieged #Aleppo today, all streets & destroyed buildings are full with dead bodies. It's hell.— The White Helmets (@SyriaCivilDef) December 13, 2016
UN Secretary-General Ban Ki-moon has voiced alarm over reports of atrocities against "a large number" of civilians and urged combatants to protect the city's residents.
Mr Ban's spokesperson said: "While stressing that the United Nations is not able to independently verify these reports, the secretary-general is conveying his grave concern to the relevant parties.
"The United Nations underlines the obligation of all parties on the ground to protect civilians and abide by international humanitarian and human rights law."
He added that this was "particularly the responsibility" of the Syrian government and Russia, which is backing Bashar al Assad's regime.
Mr Ban's plea came as Jan Egeland, UN humanitarian adviser on Syria, said Damascus and Moscow "are accountable for any and all atrocities that the victorious militias in Aleppo are now committing".
The UN Under-Secretary-General for Humanitarian Affairs, Stephen O’Brien said teams are ready to help those caught up in the fighting – as soon as they can reach them:
Between 10,000 and 13,000 civilians have fled from eastern Aleppo since Monday, bringing the total number to have escaped the fighting there to an estimated 130,000, the Syrian Observatory for Human Rights said.
According to the Russian defence ministry, 100,000 civilians are left in the eastern part of the city.
While Aleppo's fall would deal a stunning blow to rebels, Mr Assad would still be far from restoring control across Syria. Swathes of the country remain in rebel hands, and Islamic State retook Palmyra on Sunday.