There are fears of a genocide in the city of Aleppo after it fell to the Syrian Army last night.
Local media claim civilians have been lined up and shot in mass executions.
Aleppo24, which describes itself as an independent news source, claims there has been at least one incident of children being burned alive.
President Assad's forces appear to have taken control of Aleppo which has been at the centre of the country's bloody civil war for the past four years.
There are now concerns for up to 100,000 civilians in the city who fear reprisals as the Syrian Army takes control from rebel forces.
They are said to be trapped in an area no more than five miles wide and are calling on the UN to facilitate their passage to safety.
Many have been tweeting heartbreaking final messages calling for help.
Lina Shamy, an activist tweeting from inside Aleppo, posted a video message on Twitter saying: "To everyone who can hear me.
The Secretary General of the United Nations Ban Ki Moon has said he is "alarmed" by the reports of atrocities.
There has been no official response from the Syrian government or its ally Russia on the cliams.
The White Helmets, a group of volunteer Syrians who act as first responders, has published an impassioned plea for the UN to intervene.
In a message the group wrote: "The bombs are falling as we write this. For years our humanitarian volunteers have worked to save the lives of our people in Aleppo: operating in underground hospitals, rescuing entire families buried under rubble and risking our lives to document what the daily war crimes committed by Assad regime and its ally Russia. We can do no more.
"Now we are with 100,000 civilians trapped in an area of five square miles with non-stop bombs, shells and advancements on the ground. In one building more than 500 people are sheltering. People have been underground for days."
"We are calling on you in the International Community to provide safe passage out of Aleppo for the remaining 100,000 people. We know that the uN has a plan to get us out across the 4 kilometres of Western Aleppo to safety: with a few dozen buses and lorries we could all be evacuated in twenty-four hours. However, we need you to guarantee the safety of their workers and our own.
"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. We’ve watched thousands of our boys and men be detained. Countless White Helmets, doctors, nurses and humanitarians have been targeted and killed in the regime’s cruel assault on Aleppo. The regime has been trying to kill us for five years. Please don’t get them this chance.
"We cannot believed that the world’s most powerful countries cannot get 100,000 souls four kilometres to relative safety."
"The Secretary-General is conveying his grave concern to the
Crowds of civilians have been seen celebrating the advances in Assad-held sectors of the city, waving their country's national flag in the air along with portraits of the president.
"He has instructed his Special Envoy for Syria to follow up urgently with the parties concerned," UN spokesman Stephane Dujarric said.
Millions of people have fled Syria during the civil war into Europe and elsewhere.
An estimated 10,000 have fled in last 24 hours alone.
The US and Russia made attempts over the weekend to agree on a ceasefire to allow civilians to leave but failed.
The UN humanitarian adviser on Syria, Jan Egeland said in a tweet, that the Syrian and Russian governments must be held responsible.
"The Gov'ts of Syria & Russia are accountable for any and all atrocities that the victorious militias in Aleppo are now committing!," Egeland tweeted, as Assad's forces bombarded the last rebel-held pocket of besieged eastern Aleppo.
Rebels withdrew from all districts on the east side of the Aleppo river on Monday afternoon after losing Sheikh Saeed in the south of their pocket in overnight fighting
"The battle in eastern Aleppo should end quickly.
"The (rebels) don't have much time. They either have to surrender or die," Lieutenant General Zaid al-Saleh, head of the government's Aleppo security committee, told reporters in the recaptured Sheikh Saeed district of the city.
British ministers will come under pressure today to help rescue civilians.
Former Cabinet minister Andrew Mitchell has secured an emergency debate on the humanitarian crisis and will urge the UK to use its "immense diplomatic muscle" to help secure a ceasefire in the besieged Syrian city.
Commons Speaker John Bercow granted him an emergency debate on the issue, which will see ministers forced to respond in Parliament.
The debate was granted shortly after Defence Secretary Sir Michael Fallon warned it is "almost impossible" to use air drops to get aid into the city while Russia controls the air defences in Syria.
Mr Mitchell said: "The debate would enable us to explore with the Government how Britain's immense diplomatic muscle, the finest foreign service in the world, can do more to secure a deal that will ensure a ceasefire for at least 24 hours to enable innocent civilians to be rescued from the hideous circumstances which now prevail in east Aleppo."
He cited reports that sarin and chlorine gas have been used in the city, which would constitute a war crime, as evidence of the need for immediate action.
He said: "Many of these terrified civilians trapped in this hell hole, which now resembles Stalingrad at the end of its destruction, are children.
"They have few places to hide."
With temperatures plummeting to below freezing in Aleppo, Mr Mitchell said it was not a question that "something must be done" but rather "what in the name of humanity we the international community will do to save those who today are in such dreadful jeopardy".
Foreign Secretary Boris Johnson has described the situation in Aleppo as "dire" and demanded that the Assad regime and its backers allow humanitarian access.