The Syrian army announced the start of a new military offensive in the rebel-held east of the city of Aleppo on Thursday, signalling a potential further escalation in a bombing campaign by Russian and Syrian jets that rebels say has already intensified in the last 24 hours.

State media quoted the army's military headquarters in Aleppo as urging civilians in eastern parts of the city to avoid areas where "terrorists" were located and saying it had prepared exit points for those who wanted to flee, including rebels.

The army announcement did not say whether the campaign would also include a ground incursion.

Rebels said Russian and Syrian jets had escalated strikes in the past 24 hours on areas that control access to the city, whose rebel-held east has been besieged by Russian and Iranian-backed pro-government forces and completely encircled for all but a few weeks since July.

The Syrian Observatory for Human Rights also confirmed the intensification of raids on residential areas in eastern Aleppo, saying at least 14 people had been killed on Thursday, mostly civilians.

The United States and Russia have been leading diplomatic efforts to negotiate a lasting ceasefire and have been discussing how to coordinate attacks on militants from the Islamic State and the group formally known as the Nusra Front.

A seven-day-old ceasefire brokered by Washington and Moscow collapsed last week.-Reuters