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.
Despite the apparent setback, state media reported the departure of buses carrying people from Aleppo.
This has not been confirmed by other sources.
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.
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.
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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.
Rebel 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 rebel 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.
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.
“We are extremely concerned about their fate. If these children are not evacuated urgently, they could die.”
France has proposed that UN officials should monitor evacuation efforts and report on the protection of civilians. The UN will vote on Sunday on whether to send observers to the stricken city.
But there are concerns that the motion may be resisted by Russia, an ally of Syria’s president and a veto-wielding Security Council member.
Moscow has vetoed six resolutions on Syria since the conflict began in 2011.