A bomb blast hit a bus convoy waiting to enter Aleppo on Saturday, killing and wounding dozens of people after an evacuation deal between Syria's warring sides halted and stranded thousands at two transit points on the city outskirts.
Pro-Damascus media outlets said a suicide attacker had detonated a car bomb and killed at least 22 people. Images posted by the outlets showed bodies lying next to charred buses with their windows blown out, and flaming vehicles belching out thick black smoke.
British-based monitor the Syrian Observatory for Human Rights counted 24 dead and dozens more wounded.
The blast hit buses in the Rashidin area on Aleppo's outskirts, which had been waiting to cross from opposition-held territory into the government-controlled city itself, carrying people evacuated from two Shia villages on Friday.
The residents, alongside hundreds of pro-government fighters, had left the two opposition-besieged villages in northwest Idlib province under a deal where in exchange, hundreds of Sunni insurgents and their families moved out of a government-besieged area near Damascus.
But a delay in the agreement had left all those evacuated stuck at two transit points on Aleppo's outskirts since late on Friday.
Opposition forces and residents of Madaya near Damascus were waiting at the government-held Ramousah bus garage, a few miles away. They were to be transported to Idlib province, which the armed opposition controls. The agreement is one of several over recent months that has seen President Bashar al-Assad's government take back control of areas long besieged by his forces and their allies. The Observatory said the delay was caused by the fact that opposition forces from Zabadani, another town near Damascus included in the deal, had not yet been granted safe passage out.
Syria's opposition says the evacuation deals, which include areas of Aleppo and a district in the city of Homs, amount to forced displacement of Assad's opponents from Syria's main urban centres in the west of the country. The agreements are also causing demographic changes, they say, because those who are displaced are mostly Sunni Muslims.
Syria's population is mostly Sunni. Assad is from the Alawite religious minority, often considered an offshoot of Shia Islam.
He has been backed militarily by Russia, and by Shia fighters from Iran and the Lebanese Hezbollah group in Syria's six-year-old conflict.