Several buses en route to evacuate the sick and injured from two government-held villages in Syria’s Idlib province have been burned by rebels.
The convoy was travelling to Foah and Kefraya, besieged by rebel 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.
Thousands of people are waiting to leave in desperate conditions, reports 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.
Despite delays caused by disagreements over the new evacuation plan, convoys were said to be travelling to both eastern Aleppo and the government-held villages in Idlib province on Sunday.
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.
It had reported earlier that the rebel group Jabhat Fatah al-Sham, formerly known as the Nusra Front, was preventing buses entering the villages.
Syrian state media said “armed terrorists” attacked five buses, burned and destroyed them.
Rebel groups have not yet commented on the attack.