Armed men burned five buses that were supposed to be used for an evacuation near Idlib in Syria today, stalling a deal to allow thousands to depart the last rebel pocket in Aleppo, where evacuees crammed into buses for hours, waiting to move.

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

Hours after the incident, as the Aleppo evacuees waited on their buses, it was still unclear what impact the bus burning would have on the wider agreement.

However, Russian President Vladimir Putin, Assad's main foreign backer, and Turkish President Tayyip Erdogan, the rebels' main supporter, agreed by telephone on Sunday that the disruptions must be quickly overcome, sources in Erdogan's office said.

The commander of forces allied to Assad said there was still a chance for states with influence over rebel groups to find a way to evacuate civilians safely.

In a statement carried by a military news outlet run by Damascus's ally, the Lebanese group Hezbollah, the allied forces leadership said responsibility for the delay in the evacuation falls with "terrorists and their state sponsors".

Some 40 km (26 miles) 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.

The Syrian Observatory for Human Rights said there was no sign the buses had left Aleppo or the villages, and a passenger on one told Reuters he had been on the bus for four hours and was still in the city's rebel enclave.

Syrian state television, citing its correspondent in the city, said buses had started to leave east Aleppo where over 15,000 people had gathered in a square to wait, many after a night sleeping in the streets in freezing temperatures.

Aleppo had 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.

"Everyone is waiting"

According to Syria's al-Ikhbariya TV news, about 1,200 civilians would initially be evacuated from east Aleppo and a similar number from the two villages.

A document cited by al-Manar television and passed to Reuters by rebels and activists said the entire deal would see 2,500 citizens leave al-Foua and Kefraya in two batches, in exchange for the evacuation of people from east Aleppo in two corresponding batches.

Following this, another 1,500 would leave al-Foua and Kefraya in exchange for the evacuation of 1,500 from the towns of Madaya and Zabadani near Lebanon, which are besieged by pro-government forces.

Once evacuees from the villages have safely arrived in government areas, Aleppo fighters and more of their family members will be allowed to leave, in return for subsequent batches of people departing al-Foua and Kefraya, al-Ikhbariya TV reported.

In the square in Aleppo's Sukari district, organisers gave every family a number to allow them on buses.

"Everyone is waiting until they are evacuated. They just want to escape," said Salah al Attar, a former teacher with his five children, wife and mother.

Thousands of people were evacuated on Thursday, the first to leave under a ceasefire deal that would end years of fighting for the city and mark a major victory for Assad.

They were taken to rebel-held districts of the countryside west of Aleppo. Turkey has said Aleppo evacuees could also be housed in a camp to be constructed near the Turkish border to the north.

United Nations vote

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 on Sunday on a compromise draft resolution on U.N. officials monitoring the evacuations from Aleppo. It will vote on the text on Monday.

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 9 am (1400 GMT)," US Ambassador to the United Nations, Samantha Power, told reporters after more than three hours of negotiations.

Russian Ambassador Vitaly Churkin said: "I think we have a good text, we agreed to vote tomorrow morning."

Russia, which has provided military backing to Assad, has vetoed six Security Council resolutions on Syria since the conflict started in 2011. China joined Moscow in vetoing five resolutions.

A crackdown by Assad on pro-democracy protesters in 2011 led to civil war, and Islamic State militants have used the chaos to seize territory in Syria and Iraq. Half of Syria's 22 million people have been uprooted and more than 400,000 killed.