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 traveling 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.
Despite delays over the new operation, buses were preparing to evacuate people from both eastern Aleppo and the government-held villages in Idlib province on Sunday.
A number of buses have now succeeded in entering Foah and Kefraya, according to the UK-based monitoring group the Syrian Observatory for Human Rights (SOHR). It earlier reported that Jabhat Fatah al-Sham, formerly al-Qaeda-linked Nusra Front, had been holding them up.
However, six buses were attacked and torched on the way, the SOHR said.
Syrian state media said "armed terrorists" attacked five buses, burned and destroyed them.
Several reports from opposition sources said Jabhat Fatah al-Sham was responsible. But Hezbollah's al-Manar TV and Beirut-based pro-Syrian government al-Mayadin TV said clashes between different jihadist groups had resulted in the blaze.
The initial plan to evacuate the last rebel-held enclaves in eastern Aleppo collapsed on Friday, leaving civilians stranded at various points along the route out without access to food or shelter.
The UN Security Council is due to vote on Sunday on French proposals for UN officials to monitor the evacuations of besieged neighbourhoods in the city.
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 rebel enclave in return for a similar number moved out of the two government-held villages, Foah and Kefraya.
Among the people waiting to leave eastern Aleppo are sick and wounded children, said the children's charity Unicef.
Some young children have been forced to leave without their parents, the charity said, and hundreds of vulnerable children remain trapped.
What is going to happen to the jihadists and their families?
Why is it the civilians who are being forced to leave - it is the rebels who should be made to leave, especially as many of them are not even Syrian
The civilians should not be made to leave and freeze to death in tents - these poor unfortunate people have held out for many months waiting for someone to free them from jihads and Sharia law - they need their city and immediate resources to feed and rebuild the city
But think - did the coalition rush in to rebuild Kobane after they happily bombed it to oblivion?
NO THEY DID NO
In fact many BILLIONS of dollars have been spent on the destruction - almost NOTHING is being spent on rebuilding Syria
