The Syrian army and its allies are in the "last moments before declaring victory" in Aleppo, a Syrian military source said, after rebel defenses collapsed on Monday, leaving insurgents in a tiny, heavily bombarded pocket of ground.
A Reuters journalist in the government-held zone said the bombardment of rebel areas of the city had continued non-stop on Monday, 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 Monday 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 inTurkey.
The White Helmets civil defense organization 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. (1900 GMT).
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 defense 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.
"People run from one shelling to another to escape death and just to save their souls ... It's doomsday in Aleppo, yes doomsday in Aleppo," said Abu Amer Iqab, a former government employee in the Sukkari district in the heart of the rebel enclave.
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.
While Aleppo's fall would deal a stunning blow to rebels trying to remove Assad from power, he would still be far from restoring control across Syria. Swathes of the country remain in rebel hands, and on Sunday Islamic State retook Palmyra.
Tens of thousands of civilians remain in rebel-held areas, hemmed in by ever-changing front lines, pounded by air strikes and shelling, and without basic supplies, according to the Observatory, a British-based monitoring group.
In the Sheikh Saeed district, an elderly couple stood lamenting their fate.
"May every son return to his mother. I have suffered that loss. May other women not endure the same," said the woman, her arms raised to the sky. "I have lost my three children. Two died in battle and the third is kidnapped," she added, as an army officer attempted to calm her.
Rebel groups in Aleppo received a U.S.-Russian proposal on Sunday for a withdrawal of fighters and civilians from the city's opposition areas, but Moscow said no agreement had been reached yet in talks in Geneva to end the crisis peacefully .
The U.S. State Department said Russia had turned down the U.S. proposal for an immediate ceasefire and said it could not start for several days.
The rebel official said Moscow had no incentive to compromise while Assad was gaining ground. "The Russians are being evasive. They are looking at the military situation. Now they are advancing," he said.
The U.S. National Security Council also said, in a message passed on by the American mission in Geneva, that Moscow had rejected a ceasefire. "We proposed an immediate cessation of hostilities to allow for safe departures and the Russians so far have refused," it said in a statement.
The Syrian army is backed by Russian war planes and Lebanese and Iraqi Shi'ite militias supported by Iran. Its advances on Monday were aided by a militia of Palestinian refugees in Syria, the Liwa al-Quds or Jerusalem Brigade, the general said.
The mostly Sunni rebels include groups backed by the United States, Turkey and Gulf monarchies as well as hardline jihadists who are not supported by the West.
A correspondent for Syria's official SANA news agency said the army had taken control of Sheikh Saeed, and more than 3,500 people had left at dawn.
A Syrian official told Reuters: "We managed to take full control of the Sheikh Saeed district. This area is very important because it facilitates access to al-Amariya and allows us to secure a greater part of the Aleppo-Ramousah road." The road is the main entry point to the city from the south.
Riad Hijab, Syria's chief opposition coordinator, said the rebels' defeat in Aleppo would not weaken the resolve of Assad's opponents, or push them to water down the demand that he quit.
"If Assad and his allies think that a military advance in certain quarters of Aleppo will signify that we make concessions, then (I say) that will not happen," he told reporters after meeting French President Francois Hollande.
The loss of Palmyra, an ancient desert city whose recapture from Islamic State in March was heralded by Damascus and Moscow as vindicating Russia's entry into the war, is an embarrassing setback to Assad.
The Observatory reported that the jihadist group carried out eight executions of Syrian soldiers and allied militiamen in Palmyra on Monday while warplanes bombarded their positions around the city.
Still, Islamic State made further advances around Palmyra, it and the Observatory said on Monday, including coming close to a military air base.
Another four people, including two children, were shot dead while the jihadists cleared the city, the Observatory said.
It said at least 34 people had died in air raids on an Islamic State-held village north of Palmyra, and that local officials said poison gas had been used. Islamic State accused Russia of the attack. Both Russia and Syria's military deny using chemical weapons.
The Russian Defence Ministry said on Monday that 728 rebels had laid down their weapons over the previous 24 hours and relocated to western Aleppo. It said 13,346 civilians left rebel-controlled districts of Aleppo over the same period.
"Displaced people are moving," said an Aleppo resident. Some were moving from areas controlled by the army to opposition areas, while others were going in the opposite direction. Some were staying at home waiting for the army.
The Observatory said that four weeks into the army offensive at least 415 civilians, including 47 children, had been killed in rebel-held parts of the city.
The Observatory said 364 rebel fighters had been killed in the eastern sector. It said rebel shelling of government-held west Aleppo had killed 130 civilians including 40 children.