Several buses en route to transport the sick and injured from two government-held villages in Syria’s Idlib province have been burned by rebels.
It has thrown the latest efforts to evacuate besieged areas into doubt.
Pro-government forces say people must be allowed to leave the mainly Shia villages of Foah and Kefraya for the evacuation of east Aleppo to restart.
State media said convoys had begun to leave Aleppo on Sunday but other reports said they had turned back.
The initial plan to evacuate the last rebel-held enclaves in the city collapsed on Friday, leaving civilians stranded at various points along the route out without access to food or shelter.
Russia, which backs the government of President Bashar al-Assad, says it will veto a French-drafted resolution to send UN officials to monitor the evacuations in Aleppo. A vote is due at the UN Security Council on Sunday.
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 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.
A reporter for AFP news agency said armed men forced the drivers of the buses to get out before opening fire and setting the vehicles light.
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.
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