|Supporters of Syria's President Bashar al-Assad carry their national flags and gesture as they tour the streets in celebration of what they say is the Syrian army's victory against the rebels in Aleppo, Syria December 12, 2016. REUTERS/Omar Sanadiki
A Reuters journalist in the government-held zone said the bombardment of rebel areas of the city had continued non-stop on December 12, and a civilian trapped there described the situation as resembling "Doomsday".
"The battle in eastern Aleppo should end quickly. They (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.
Rebels withdrew from all districts on the east side of the Aleppo river on December 12 afternoon after losing Sheikh Saeed in the south of their pocket in overnight fighting, the Syrian Observatory for Human Rights said.
In government-held districts soldiers started to fire into the air in celebration, carving tracer bullets into Aleppo's night sky, and car drivers honked their horns at what they believed was imminent victory, state television showed.
The rebels' rapidly diminishing enclave had halved in only a few hours and Observatory director Rami Abdulrahman described the battle for Aleppo as having reached its end.
"The situation is extremely difficult today," said Zakaria Malahifji of the Fastaqim rebel group fighting in Aleppo.
An official from Jabha Shamiya, a rebel faction that is also present in Aleppo, said the insurgents might make a new stand along the west bank of the river.
"It is expected there will be a new front line," said the official, who is based in Turkey.
The White Helmets civil defence organisation and three other trapped aid groups made a desperate appeal for 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 time stamped 9 p.m. (2 p.m. ET).
Activists and two residents inside the remaining rebel enclave said at least 79 civilians were summarily executed in the Fardous and Saliheen districts by pro-government militias.
"There are more than 100 corpses and others who could be still alive under the rubble whom no one is able to get to," said civil defence chief Ammar al Selmo.
United Nations Secretary-General Ban Ki-moon was alarmed by unverified reports of atrocities against a large number of civilians, including women and children, Ban's spokesman said.
Jan Egeland, the U.N. humanitarian adviser on Syria, tweeted that Russia's and Syria's governments would be responsible for any such abuses.
"The Gov'ts of Syria & Russia are accountable for any and all atrocities that the victorious militias in Aleppo are now committing!," Egeland wrote.
The rebels' sudden retreat represented a "big collapse in terrorist morale", a Syrian military source said.
Syrian President Bashar al-Assad, backed by Russia, is now close to taking back full control of Aleppo, which was Syria's most populous city before the war and would be his greatest prize so far after nearly six years of conflict.
"We are in the final moments before declaring the victory of the Arab Syrian army in the battle of East Aleppo. We could announce this any moment," the military source told Reuters.
The Russian Defence Ministry 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 militants.
State television footage from Saliheen, one of the districts that had just fallen to the army, showed mounds of rubble and half-collapsed buildings, with bodies still lying on the ground and a few bewildered civilians carrying children or suitcases.