The Syrian army and its allies announced the capture of a large swath of eastern Aleppo from rebels on Monday in an accelerating attack that threatens to crush the opposition in its most important urban stronghold.
Two rebel officials said the insurgents, facing fierce bombardment and ground attacks, had withdrawn from the northern part of eastern Aleppo to a more defensible front line along a big highway after losses that threatened to split their enclave.
The Syrian Observatory for Human Rights said the northern portion of eastern Aleppo lost by the rebels amounted to more than a third of the territory they had held, calling it the biggest defeat for the opposition in Aleppo since 2012.
Thousands of residents were reported to have fled. A rebel fighter reached by Reuters said there was "extreme, extreme, extreme pressure" on the insurgents.
Part of the area lost by the rebels was taken over by a U.S.-backed Kurdish militia from another part of Aleppo in what rebels described as an agreed handover, a rare example of cooperation between groups that have fought each other.
Hundreds of miles to the south, people started to leave the rebel-held Damascus suburb of Khan al-Shih for other parts of the country controlled by insurgents under a deal with the government, the Observatory said.
It is the latest such agreement, characterised by Damascus as "reconciliations" but decried by rebels as the forcible removal of the populations of opposition areas, and a model that the government has suggested could be employed in east Aleppo.
Capturing eastern Aleppo would be the biggest victory for President Bashar al-Assad since the start of the uprising against him in 2011, restoring his control over the whole city apart from a Kurdish-held area that has not fought against him.
For Assad, taking back Aleppo would shore up his grip over the main population centres of western Syria where he and his allies have focused their firepower while much of the rest of the country remains outside their control.
It would be seen as a victory for his allies, Russia and Iran, which have outmanoeuvred the West and Assad's regional enemies through direct military intervention.
"What happened in the last two days is a great strategic accomplishment by the Syrian army and allies," a fighter with a militia on the government side in the Aleppo area said.
Rebels say their foreign patrons including the United States have abandoned them to their fate in Aleppo.
Assad, whose backers also include the Lebanese group Hezbollah, has gradually closed in on eastern Aleppo this year, first cutting the most direct lifeline to Turkey before fully encircling the east, and launching a major assault in September.
A military news service run by Hezbollah declared the northern portion of eastern Aleppo under full state control.
The Russian Defence Ministry said about 40 percent of the eastern part of the city had been "freed" from militants by Syrian government forces.
Russian President Vladimir Putin discussed the Syrian army