Several buses en route to evacuate ill and injured people from the besieged Syrian villages of al-Foua and Kefraya were attacked and burned on Sunday, the Syrian Observatory for Human Rights and Syrian state television said.
Some buses, as well as Red Crescent vehicles, reached the entrance to the villages in Idlib province, which are besieged by insurgents.
The coalition of forces fighting for the government of President Bashar al-Assad are demanding people to be allowed to leave the two villages in exchange for allowing evacuations of rebels and civilians from east Aleppo.
Syrian state media said "armed terrorists" - a term it uses for insurgent groups fighting against Assad's rule - attacked five buses and burned and destroyed them.
Rebel officials said an angry crowd of people, possibly alongside pro-government forces, carried out the attack.
A resident in the area told Reuters it was not carried out by the group formerly known as the Nusra Front, which had previously said it had not agreed to the evacuation of the two villages.
Most of al-Foua and Kefraya's residents are Shi'ite Muslims.