UN chief Ban Ki-moon has expressed alarm over reports of atrocities against civilians, as the battle for Aleppo entered its final phase with Syrian government forces on the verge of retaking all remaining rebel-held areas of the city.
"The secretary-general is alarmed over reports of atrocities against a large number of civilians, including women and children, in recent hours in Aleppo," his spokesman Stephane Dujarric said on Tuesday.
"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."
Activists in Aleppo have reported summary executions as pro-government forces enter districts once under control of the opposition.
The UK-based Syrian Observatory for Human Rights says at least 60 opposition members have been killed.
Jan Egeland, the UN humanitarian adviser on Syria, tweeted that Russia's and Syria's governments would be responsible for any abuses.
"The opposition is describing the situation in besieged areas as catastrophic," Al Jazeera's Zeina Khodr, reporting from Antakya in southern Turkey, said.
"They are issuing an urgent appeal to evacuate civilians who are trapped in besieged areas," she said. "Tens of thousands of people are believed to be in a very small pocket of territory coming under intense bombardment.
"There have been desperate messages from people trapped inside asking, 'Where is humanity?', 'Where is the world?', 'Why is nobody helping us?'"
|Tens of thousands of people remain in rebel-held areas of Aleppo [Reuters]
The fall of Aleppo would be the biggest rebel defeat since Syria's conflict began in 2011, and leave the government in control of the country's five major cities.
Government victory imminent
"The rebels in Aleppo have all but declared defeat. We understand that there is no agreement for any evacuation plan. The United States says negotiations in Geneva reached an impasse; Russia says it reached a dead end. Our sources in the Free Syrian Army say Russia is demanding the rebels surrender, hand themselves over to pro-government forces - that would be the condition for a ceasefire and to create a safe corridor for civilians to leave... Opposition activists, citizen journalists, and White Helmet volunteers are also there [in opposition-held areas of Aleppo] and according to the government, engaging in treason. They fear for their safety - for their lives - if they are caught by government forces."
According to the AFP news agency, a Syrian military official in Aleppo said late on Monday: "We're living the final moments before victory."
"The operation in eastern neighbourhoods is entering its final phase," he said earlier, as fierce clashes were reported in the few districts still under rebel control.
The pro-government TV network Alikbariyah Syria broadcast footage of celebrations in the streets of west Aleppo, as people handed out chocolates and congratulated each other on "the victory".
"The joy of the people and the army are one," said one driver, wearing military fatigues.
READ MORE: Hundreds of men from east Aleppo go 'missing'
In the Mashhad neighbourhood, residents fleeing the army advance crowded the streets earlier on Monday, AFP quoted witnesses as saying.
Displaced civilians - many hungry after fleeing without food - sat on pavements or lay on the street with nowhere else to go, they said.
The Syrian Observatory said on Monday that another 10,000 people had fled rebel areas in the previous 24 hours, bringing the total number of those who have left - mostly to government-held territory - to 130,000.
The Russian defence ministry also said that since the start of the Aleppo battle, more than 2,200 rebels had surrendered and 100,000 civilians had left areas of the city that were controlled by the opposition.
"The battle of Aleppo has reached its end. It is just a matter of a small period of time ... it's a total collapse," said the director of the Syrian Observatory, Rami Abdul Rahman.
But while Aleppo's fall would deal a stunning blow to opposition fighters trying to remove Assad from power, he would still be far from restoring control across Syria.
Swaths of the country remain in rebel hands, while the Islamic State of Iraq and the Levant group (ISIL, also known as ISIS) reportedly retook the ancient city of Palmyra on Sunday.
Source: Al Jazeera and news agencies