A deal to evacuate trapped civilians and fighters in war-ravaged east Aleppo and two Syrian villages has been thrown into doubt after assailants torched six buses assigned to the operation.
The buses were to take part in the evacuation of over 2,000 wounded and sick Syrians from Foua and Kfarya, two rebel-besieged villages that have remained loyal to the government in an area under opposition control in the north-west Idlib province.
The bus attacks could scuttle a wider deal to evacuate thousands of vulnerable civilians and fighters from the opposition's last foothold in Aleppo and return the city entirely to government control.
Evacuations from Aleppo had been halted amid mutual recriminations on Friday after several thousand trapped civilians had already been moved from the city.
The suspension of the evacuations had thrown an Aleppo deal brokered by Russia and Turkey last week into disarray.
That deal marked a turning point in the country's war. With the opposition leaving Aleppo, President Bashar Assad has effectively reasserted his control over Syria's five largest cities and its Mediterranean coast nearly six years after a national movement to unseat him took hold.
The opposition's Britain-based Syrian Observatory for Human Rights said the al Qaida-affiliated Fatah al-Sham Front was behind Sunday's assault on the buses. The insurgent group had been dragging its feet over approving the evacuation deal.
Hezbollah, the Lebanese militant group fighting alongside Syria's government, said the buses were burned during fighting between Fatah al-Sham and a rebel group that supported the evacuations.
Most residents of the two villages are Shiite Muslims, while the most powerful anti-government groups in Idlib are hard-line Sunnis.
The identity of the group behind the attack remains unclear. A video showing armed men circling the burning buses did not reveal their affiliation.
"The buses that came to evacuate the apostates have been burned," declared the narrator of the video, as celebratory gunshots rang through the air. He warned that no "Shiite pigs" would be allowed to leave the towns.
A coalition of rebel groups condemned the bus burning as a "reckless attack," saying it endangered tens of thousands of Syrians trapped in Aleppo. No group has claimed responsibility for burning the buses.
Earlier in the day, dozens of buses and ambulances were poised to enter east Aleppo to resume evacuating rebel fighters and civilians from the opposition's remaining districts, pro-Syrian government media said. But the evacuations remained on hold at nightfall.
The International Committee of the Red Cross, which has overseen the Aleppo evacuations, had no comment on their possible resumption. The agency has said thousands of people, among them women, children, the sick and the wounded, remain trapped in besieged areas of the city, waiting in freezing temperatures for the evacuations to resume.
Also on Sunday, the UN Security Council was expected to vote on a resolution demanding immediate and unconditional access for the United Nations and its partners to besieged parts of Aleppo and throughout Syria to ensure the delivery of humanitarian aid.
The council began consultations on the French-drafted resolution followed by an open meeting where members are expected to vote.
The draft resolution calls on Secretary-General Ban Ki-moon to immediately redeploy UN humanitarian staff already on the ground to carry out "neutral monitoring" and "direct observation and to report on evacuations".
It stressed that evacuations of civilians must be voluntary and to the destinations of their choice.
Russia's UN ambassador Vitaly Churkin said on Friday he would examine the draft, but was sceptical that monitors could be deployed quickly.