The United Nations says it has reports of Syrian government troops and their allies killing civilians "on the spot" in Aleppo as they close in on recapturing the city from rebels.

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 army said its troops and allied forces 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".

Amid reports of atrocities and a humanitarian disaster, many from inside the city posted what they thought were their final messages.

Seven-year-old Bana Alabed, whose tweets during the conflict have captivated the world, wrote: "My name is Bana, I'm 7 years old. I am talking to the world now live from East #Aleppo. This is my last moment to either live or die. - Bana".

The UN human rights office said it had reports of Syria pro-government forces killing at least 82 civilians in the eastern part of Aleppo, which has been held by rebels - including 11 women and 13 children, in four different neighbourhoods.

"The reports we had are of people being shot in the street trying to flee and shot in their homes," UN humanitarian spokesman Jens Laerke told reporters in Geneva.

He said the situation looked like "a complete meltdown of humanity in Aleppo".

And UNICEF said 100 children are trapped in a building that has come under heavy attack.

Rupert Colville, spokesman for the UN High Commissioner for Human Rights, said the reports recount pro-government forces entering homes and killing some civilians "on the spot" in the former rebel enclave.

"We're filled with the deepest foreboding for those who remain in this last hellish corner," he said.

Saying it was increasingly difficult to contact people inside the city, he called the situation "terrifying".

"Literally nowhere safe to run"

The UN had received "reports of bodies lying in the streets and people unable to retrieve them because of the intensity of the bombardment and fear of being shot", Mr Colville said.

On Monday, UN secretary-general Ban Ki-moon voiced alarm over reports of atrocities against "a large number" of civilians and urged combatants to protect the city's residents.

He said this was "particularly the responsibility" of the Syrian government and Russia.

Aleppo has been split since 2012, becoming a key battlefield in Syria's civil war as well as the epicentre of a humanitarian crisis. Mr Assad's forces launched a new offensive to retake it last month.

Inside the besieged city, residents sent texts, tweets and videos to make desperate 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".

With rain falling in the background, Mr Alhamdo struggled to fight back tears as he described the devastation.

"Just yesterday, next door all the building collapsed and many people were killed," he said.

"You might not understand what we are suffering here," he said.

"We wanted freedom, we didn't want anything else but freedom," he goes on to say.

"I hope that something can be done and that we can leave and that we can speak together once more...I hope you can remember us."

The International Committee of the Red Cross said thousands of people with no part in the violence "have literally nowhere safe to run".

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 4-km (2.5-mile) 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.

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.