A bomb blast has hit a bus convoy waiting to cross into government-held Aleppo in Syria, killing and wounding dozens of people evacuated from two Shi'ite villages the day before in a deal between warring sides.
The agreement had stalled, leaving thousands of people from both government-besieged and rebel-besieged areas stranded at two transit points on the city's outskirts, before the explosion occurred.
Late on Saturday buses from the convoy hit by the bombing began crossing into government-held territory as the deal resumed, pro-Damascus media outlets reported.
But the incident underscored the difficulty carrying out any agreement between warring sides in a volatile and complex Syrian conflict which, in its seventh year, shows no signs of easing.
A media unit run by Damascus ally Hezbollah said the attack was carried out by a suicide car bomb and killed at least 40 people. The Observatory said more than 24 were killed.
Footage on state TV showed bodies lying next to charred buses with their windows blown out, and vehicles in flames.
The blast hit buses in the Rashidin area on Aleppo's outskirts. The vehicles had been waiting since Friday to cross from rebel-held territory into the government-controlled city itself. Ambulances later took the wounded to hospital in Aleppo.
The convoy was carrying residents and pro-government fighters from the rebel-besieged Shi'ite villages of al-Foua and Kefraya in nearby Idlib province.
They had left under a deal where, in exchange, hundreds of Sunni insurgents and their families were granted safe passage from Madaya, a government-besieged town near Damascus.
But a delay in the agreement had left all those evacuated stuck at transit points on Aleppo's outskirts since late on Friday.
Residents of al-Foua and Kefraya were waiting in the Rashidin area.
Rebels and residents of Madaya, near Damascus, were meanwhile waiting at the government-held Ramousah bus garage, a few miles away. They were to be transported to the opposition stronghold of Idlib province.
People waiting in the Ramousah garage heard the blast, and said they feared revenge attacks by pro-government forces. They circulated a statement on social media imploring "international organisations" to intervene so the situation did not escalate.