Syrian activists say at least five buses assigned to evacuate wounded and sick people in two northern villages have been burnt, threatening a wider deal to ship out thousands of civilians and rebel fighters still trapped in east Aleppo.
The UN Security Council, meanwhile, was set to vote on a French-drafted proposal to send monitors to Aleppo to observe evacuations from besieged areas.
Disagreements over the evacuation of thousands of civilians and the remaining rebels in eastern Aleppo had caused delays.
But Syrian state media said on December 18 that buses entered eastern Aleppo under the supervision of the International Red Cross and the Syrian Arab Red Crescent to begin the evacuations.
Some 1,200 people were to be taken out of the rebel enclave in return for a similar number taken out of Foah and Kefraya -- two villages in Idlib Province that have been besieged by the armed opposition.
However, several buses en route to evacuate ill and injured people from the two villages were attacked and burned, the British-based Syrian Observatory for Human Rights and Syrian state media later reported.
The group blamed Al-Qaeda-affiliated miliants.
The Security Council was set to convene early on December 18 to decide on the draft proposal despite resistance from Russia, which holds veto power and is backing Syrian President Bashar al-Assad in the war.
The text of the draft resolution asks UN Secretary-General Ban Ki-moon to "redeploy the United Nations humanitarian staff already on the ground to carry out adequate, neutral monitoring, direct observation, and to report on evacuations from besieged parts of Aleppo and protection of civilians inside Aleppo."
The proposal would also seek to ensure the "voluntary, safe, and dignified passage of all civilians" from Aleppo and other areas subject to UN monitoring, with the wounded and most vulnerable given priority.
It also asks Ban to report back to the council on the resolution's implementation within five days after it is adopted.
Thousands of civilians and the remaining rebels in Aleppo were waiting for evacuations to resume on December 17, a day after the Syrian government suspended an evacuation operation brokered by Russia and Turkey amid conflicting allegations of cease-fire violations.
Earlier on December 17, Syria's state television said that the evacuation was still suspended until rebels allowed residents of two besieged Shi'ite villages -- Fua and Kfarya -- to leave to government-controlled areas.
Assad's forces made a push this week to establish full control over the eastern part of Aleppo, which the opposition had held since 2012, with an offensive that has been harshly criticized by the UN and Western governments.
Late on December 16, France circulated a draft text of its proposed resolution stating that the Security Council was "alarmed" by the humanitarian crisis in Aleppo and due to the fact that "tens of thousands of besieged Aleppo inhabitants" require aid and evacuation.
It was not immediately clear whether Russia, Assad's key supporter in the war, would back the proposal.
Vitaly Churkin, Russia's ambassador to the UN, said that some aspects of the French proposal were "questionable," including the prospect of quickly redeploying the monitors.