The operation to evacuate people from the last rebel areas of Aleppo has started, eye witnesses have said, with long snakes of vehicles leaving the city for the neighbouring rebel-held province.

Despite reports that at least one White Helmets volunteer had been killed and three others injured by snipers while clearing rubble from the road, at least 10 ambulances and 17 buses began their journey to Idlib by lunchtime on Thursday.

Around 5,000 people, both fighters and civilians, are due to reach Idlib by nightfall in repeat trips made by the government buses.

"Thousands of people are in need of evacuation, but the first and most urgent thing is wounded, sick and children, including orphans," Jan Egeland, the UN's humanitarian adviser for Syria, told reporters in Geneva.

A new deal was struck late on Wednesday to evacuate people from the remaining slivers of rebel territory in the embattled city. It is unclear how many remain in the last rebel enclaves: the UN puts the figure anywhere between 50,000 - 100,000, but critics say the estimate is too high.

A previous ceasefire mediated by Turkey and Russia broke down after about 12 hours on Wednesday, and heavy fighting and air strikes resumed.

Aleppo before the Syrian Civil War Show all 12 1 /12 Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A man crosses a street in Aleppo, December 12, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A vendor sits inside an antique shop in al-Jdeideh neighbourhood, in the Old City of Aleppo, December 12, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A view shows part of Aleppo's historic citadel, overlooking Aleppo city, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A view shows part of Aleppo's historic citadel, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War Visitors walk inside Aleppo's Umayyad mosque, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War People walk inside the Khan al-Shounah market, in the Old City of Aleppo, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A man walks past shops in al-Jdeideh neighbourhood, in the Old City of Aleppo, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War People walk along an alley in al-Jdeideh neighbourhood, in the Old City of Aleppo, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War Visitors tour Aleppo's historic citadel, Syria December 11, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A general view shows the Old City of Aleppo as seen from Aleppo's historic citadel, Syria December 11, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War People walk near Aleppo's Bab al-Faraj Clock Tower, Syria October 6, 2010 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A man stands inside Aleppo's historic citadel, overlooking Aleppo city, Syria December 11, 2009 Reuters

The UK-based Syrian Observatory for Human Rights said that fighting stopped in the city at around 4 am local time (2 am GMT).

The Russian military's Centre for Reconcilation in Syria said it was preparing for a full rebel withdrawal from Aleppo and that their safety would be guaranteed.

The new deal will also see the rebel siege on the loyalist villages of Fouaa and Kfarya in Idlib eased, and aid and evacuations for the starving rebel towns of Madaya and Zabadani, under regime siege.

Regime-organised evacuations from besieged towns has proved an effective strategy for Syrian President Bashar al-Assad in the past. Recapturing Aleppo will be his biggest victory yet in the almost six-year-long civil war.