Several buses en route to vacate the sick and injured from two government-held villages in Syria’s Idlib area have been charred by insurgents.
The convoy was travelling to Foah and Kefraya, overwhelmed by insurgent fighters.
Pro-government forces are demanding people be allowed to leave the mainly Shia villages in order for the evacuation of east Aleppo to restart.
Despite the apparent setback, buses carrying people from Aleppo have started to leave, state media say.
The initial plan to evacuate eastern Aleppo collapsed on Friday, leaving civilians stranded at various points along the route out without access to food or shelter.
Regardless of delays caused by disagreements over the new evacuation plan, buses were due to evacuate people from both eastern Aleppo and the government-held villages in Idlib province on Sunday, according to reports.
However, UK-based monitoring group the Syrian Observatory for Human Rights said six buses were attacked and torched on the way to Foah and Kefraya.
Syrian state media said “armed terrorists” attacked five buses, burned and destroyed them.
Several reports from opposition sources said that militants from Jabhat Fath al-Sham, formerly al-Qaeda-linked Nusra Front, were responsible for setting the buses on fire. But Hezbollah’s al-Manar TV and Beirut-based pro-Syrian government al-Mayadin TV said clashes between jihadist and Islamist insurgents groups had resulted in the blaze.
Insurgent groups have not yet commented on the attack.
Syrian state media said buses entered eastern Aleppo around noon local time, under the supervision of the International Red Cross and the Syrian Arab Red Crescent.
Some 1,200 people were due to be taken out of the former insurgent enclave in return for a similar number moved out of the two government-held villages, Foah and Kefraya.
Pro-government forces had reportedly demanded that a group of people needing medical treatment also be allowed to leave the two areas.
Reports said a new agreement was reached in the early hours of Sunday but delays meant thousands of civilians remained stranded.