Syrian troops were poised to recapture all of second city Aleppo on Tuesday, in what would be the biggest blow to rebels since they launched their uprising in 2011.
UN chief Ban Ki-moon voiced alarm at reports of atrocities against civilians in Aleppo, where entire districts are in ruins and unclaimed bodies lie in the streets.
Forces loyal to President Bashar al-Assad have said the battle for the city is in its "final phase" after they seized more than 90 percent of the eastern districts which had been held by the rebels since 2012.
The turnaround in the government's fortunes has come through decisive support from its allies Iran and Russia, which has waged a bombing campaign in support of Assad since September last year.
Early on Tuesday, troops were conducting searches in newly recaptured neighbourhoods around Aleppo's Old City, the Syrian Observatory for Human Rights said.
"The regime is combing the districts of Bustan al-Qasr, Kalasseh, Fardaws, and other neighbourhoods it seized yesterday (Monday)," Observatory head Rami Abdel Rahman told AFP.
He said the regime was consolidating its control over those areas but had not made any fresh advances on Tuesday morning.
The fall of Aleppo would be the rebel's worst defeat of the civil war and put the government in control of all five of Syria's main cities.
An AFP correspondent in government-held west Aleppo said bombardment was heard overnight but had quietened by morning, when thick fog shrouded the city.
Residents of the sector gathered in the streets late into the night despite the rain, launching celebratory gunfire in a show of support for the army.
- 'Last chance' -
UN chief Ban Ki-moon expressed alarm over reports of atrocities against "a large number of civilians, including women and children" in Aleppo, his spokesman Stephan Dujarric said Monday.
"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."
The east of the city had been under army siege almost continuously since July.
On November 15, the army launched a blistering offensive to retake it with the support of Iran-backed militias.
A Syrian military official in Aleppo said late Monday: "We're living the final moments before victory. "
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Rebel fighters withdrew from six residential districts on Monday after government forces overran the key Sheikh Saeed neighbourhood on the city's southeastern outskirts.
The pullout leaves the rebels confined to just a handful of neighbourhoods in the southeast of the city, including Mashhad and part of Sukkari.
The International Committee of the Red Cross warned Tuesday that thousands of civilians were at risk "as front lines close in around them in eastern Aleppo."
"As the battle reaches new peaks and the area is plunged into chaos, thousands with no part in the violence have literally nowhere safe to run.
"This may be the last chance to save lives," it said.
- 'It's hell' -
The government assault on east Aleppo has killed at least 415 civilians since mid-November, the Observatory says. Another 130 civilians have been killed by rebel fire on the west.
Diplomatic efforts to end the conflict have repeatedly failed, and weekend talks between Russia and the United States on a ceasefire to allow the evacuation of civilians made no breakthrough.
The White Helmets emergency service, which operates in rebel-held areas across Syria, said it had been unable to pull bodies out from under collapsed buildings because of the ferocity of the bombardment.
"It's hell," it said on Twitter on Tuesday.
According to the Observatory, an estimated 130,000 people have poured out of rebel-held neighbourhoods as the army has advanced.
In the rebel-held Mashhad neighbourhood, residents fleeing the army advance crowded the streets on Monday, witnesses said.
Displaced civilians -- many hungry after fleeing without food -- sat on pavements or lay on the street with nowhere else to go.
The government's lightning advance in Aleppo has not come without cost.
While the army's firepower has been focused on recapturing it, the Islamic State group has retaken the ancient city of Palmyra from which it was driven out in a high-profile Russian-backed offensive in March.
More than 300,000 people have been killed in nearly six years of civil war. Millions more have fled their homes.