Damascus (AFP) - Syria's army announced the launch Thursday of an offensive to retake the rebel-held east of the battleground city of Aleppo, warning residents to keep away from positions held by anti-regime fighters.

"The military operations command announces the start of operations in eastern districts of the city and calls on residents to stay away from the positions of terrorist groups," it said in a statement carried by state news agency SANA, using the regime's term for the opposition.

The announcement came just minutes after the United States and Russia convened a last-ditch meeting in New York to save their Syria ceasefire plan and followed several hours of intensive bombardment that left Aleppo in flames.

The army also said it would take "all measures to facilitate the reception" of civilians and that those who arrive at its checkpoints would not be arrested.

Aleppo, Syria's second city and former economic capital, has been divided since 2012 with western districts controlled by the regime and the east in rebel hands.

The Syrian Observatory for Human Rights said it was "a large-scale land offensive supported by Russian air strikes aimed at taking bit by bit the eastern sector of Aleppo and emptying it of its residents".

The initial targets were the Amiriyah, Sukari and Sheikh Said sectors, to the south, said the monitoring group's head, Rami Abdel Rahman.

Late Thursday intense air raids battered the area.

The Observatory said the rebel areas of the town suffered the most intense strikes for several months.

An AFP correspondent in the rebel-held east of Aleppo witnessed a dozen families fleeing the Soukkari district for other rebel areas further north.

The Britain-based Observatory said earlier that bombardment and air strikes since Wednesday night targeting the rebel-held neighbourhoods of Bustan al-Qasr and Al-Kalasseh had triggered "massive fires".

The estimated 250,000 residents of east Aleppo, which rebels have held since 2012, have been living under government siege since early September.

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The conflict in Syria has cost more than 300,000 lives since 2011, during which time more than half the population have been uprooted from their homes.

The renewed heavy fighting follows the breakdown on Monday of a short-lived truce that had been brokered by Washington and Moscow.

The top diplomats from the United States and Russia were meeting Thursday with other key players in New York, after UN Secretary General Ban Ki-moon said Syria's peace process was facing a "make or break moment".