Supporters of Syria's President Bashar al-Assad celebrate the recapture of Aleppo from militant rebels Reuters

The UN Secretary-General, Ban Ki-Moon, has warned of atrocities against civilians in eastern Aleppo after the Syrian military said it had gained control of 99% of the area with rebels on the brink of defeat.

Amid reports from one Syrian TV station that the battle was over, Ban's spokesman Stephane Dujarric said "the secretary-general is alarmed over reports of atrocities against a large number of civilians, including women and children, in recent hours in Aleppo.

"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.

"He has instructed his special envoy for Syria to follow up urgently with the parties concerned," the statement added.

The UK-based monitor the Syrian Observatory for Human Rights described unconfirmed reports of dozens of opposition members being executed.

The final hours of the battle follow the Syrian Arab Army's (SAA) Russian-backed advance into the east of the city, which started in November.

Bashar al-Assad's forces on Monday (12 December) confirmed that at least 99% of the city was recaptured as insurgents from groups such as Fatah Halab (Conquest of Aleppo), the Free Syrian Army and the al-Qaeda-linked Jabhat Fateh al-Sham all retreated.

Rami Abdel Rahman, head of the UK-based monitor the Syrian Observatory for Human Rights, said: "It is just a matter of a small period of time until it's a total collapse."

Hundreds of people could be seen in the streets, celebrating, beeping car horns and waving the Syrian flag. One woman told an Aleppo TV station: "God bless the army. Congratulations to them on this victory and may god make them stronger. We're so happy."

Retaking the city would be Assad's biggest victory since the start of the Syrian Civil War in 2011. As the SAA closed in on the final quarters and recaptured the industrial Sheikh Saeed district, insurgent fighters were told to give up their arms.

"They [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 Reuters reporters.

The UN's humanitarian adviser on Syria, Jan Egeland, said in a tweet: "The governments of Syria and Russia are accountable for any and all atrocities that the victorious militias in Aleppo are now committing."

The offensive to retake insurgent-held eastern Aleppo consisted of an intensive aerial bombing campaign that knocked out most of the eastern sector's medical facilities. The eastern Aleppo rebel enclave has been cut off from outside aid since July by a government siege.

Despite the victory for Assad, the complex war, which according to the UN has killed more than 400,000 people and left half the country displaced from their homes, is far from over.