The evacuation of the rebel-held enclave of east Aleppo resumed tonight after three days of delays, a UN official in Syria said.

"Evacuations are on. Buses and ambulances are leaving east Aleppo now," the UN official said in an email message, adding that the first people left east Aleppo at around 9pm.

The official had no immediate information about a planned simultaneous operation to evacuate people from two Shi'ite villages near Idlib that are besieged by mostly Sunni rebel forces.

Earlier today the evacuations were stalled after armed men burned five buses that were supposed to be used for the evacuation near Idlib.

The Syrian Observatory for Human Rights, a British-based war monitor, said the evacuation of the villages had been postponed as a result of the fires, and that meant the evacuation of east Aleppo was also postponed.

In return for the evacuation of fighters, their families and other civilians from Aleppo, the mostly Sunni insurgents had agreed that people in the villages of al-Foua and Kefraya, Shi'ite villages that they have besieged near Idlib, should also be allowed to leave.

Videos posted on social media showed bearded men with guns cheering and shouting "God is great" after torching the green buses before they were able to reach the villages.

State media said "armed terrorists", a term it uses for all groups fighting President Bashar al-Assad, had carried out the attack. Pro-Damascus Mayadeen television blamed the rebel group formerly known as the Nusra Front.

Rebel officials said an angry crowd of people, possibly alongside pro-government "operatives", was responsible.

Some 40km to the northeast, hundreds of fighters and their families in Aleppo sat or stood in buses, hoping the evacuation would resume after a three-day hiatus. Some buses eventually began leaving at 9pm this evening.

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Aleppo has been divided between government and rebel areas in the nearly six-year-long war, but a lightning advance by the Syrian army and its allies began in mid-November following months of intense air strikes, forcing the insurgents out of most of the rebel-held territory within a matter of weeks.

The chaos surrounding the evacuation reflects the complexity of Syria's civil war, with an array of groups and foreign interests involved on all sides.

The United Nations Security Council agreed tonight on a compromise draft resolution on UN officials monitoring the evacuations from Aleppo. It will vote on the text tomorrow.

Russia said it would veto an earlier draft by France but circulated an alternative version.

"We expect to vote unanimously for this text tomorrow at 9am (2pm GMT)," US Ambassador to the United Nations, Samantha Power, said after negotiations.