BEIRUT (Reuters) - 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Two rebel officials said the insurgents, facing fierce bombardment and ground attacks, had withdrawn from the northern part of eastern Aleppo to a more defensible front line along a big highway after losses that threatened to split their enclave.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.

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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’s advances with members of his Security Council on Monday, Russian news agencies quoted Kremlin spokesman Dmitry Peskov as saying.

Officials with two Aleppo rebel groups said rebels had withdrawn to areas they could more easily defend, particularly after losing the Hanano housing complex area on Saturday.

“It is a withdrawal for the sake of being able to defend and reinforce the front lines,” an official in the Jabha Shamiya rebel group told Reuters.

KURDISH MILITIA

Citing a military source, Syrian state TV said the army and its allies had seized another key eastern Aleppo neighbourhood, al-Sakhour. The Jabha Shamiya official said part of it had been evacuated so it would become part of the new frontline.

The Kurdish YPG militia which controls the Sheikh Maqsoud district of Aleppo moved into at least two of the areas left by rebel forces. The YPG, a U.S. ally in the war with Islamic State, has been hostile to the Aleppo rebel groups.

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Rebel officials said the YPG had moved into areas on Sunday night in an agreement with the insurgents.

While some of the rebels in Aleppo have received support from states such as Turkey, Saudi Arabia, Qatar and the United States during the war, they say their foreign backers have failed them as Assad and his allies unleash enormous firepower.

“The situation is very bad and the reason is the round the clock shelling with all types of weapons,” said Abdul Salam Abdul Razaq, military spokesman for the Nour al-Din al-Zinki group, one of the main Aleppo rebel factions.

“There is very fierce fighting going on now and the regime and its supporters are destroying whole areas to allow themselves to advance,” he told Reuters. Another fighter said there was heavy attrition in “people and ammunition”.

Aleppo’s Civil Defence group of rescue workers operating in rebel areas said the only fuel it had left was what remained in its vehicles and equipment.

The fighting has forced thousands of residents of eastern Aleppo to flee. Some have gone to the Kurdish-held Sheikh Maqsoud district, others have gone over to government territory, and others have moved deeper into remaining rebel-held areas.

The Syrian Arab Red Crescent registered 4,000 people in the government-held Jibreen district of western Aleppo after they fled the rebel-held east in recent days, the UN’s humanitarian spokesman Jens Laerke said on Monday.

Mohammad Sandeh, a member of the opposition city council of Aleppo, told Reuters that many people were on the move but staying in the remaining rebel-held parts of Aleppo.

“The bombardment is still heavy. There’s big displacement from the eastern neighbourhoods. They are going towards the areas that are somewhat further (from the front lines),” he said. “There’s fear that the regime will advance more.”

Saleh Muslim, joint head of the Syrian Kurdish PYD party, told Reuters that between 6,000 to 10,000 people had fled to Sheikh Maqsoud, where they were being received.

The Observatory said several thousand more had crossed front lines in other parts of eastern Aleppo and had been taken to government-controlled areas of western Aleppo.

A spokesman for the U.N. refugee agency UNHCR said exact figures for the number of displaced were difficult to gauge, adding that more than 2,000 had left eastern Aleppo for the government-controlled district of Jibreen.