At least 43 people were killed after a suicide vehicle bomb blast hit a transit point where thousands of Syrians were awaiting evacuation from two besieged government-held towns west of Aleppo, a monitoring group reported.
The British-based Syrian Observatory for Human Rights said 24 people had been killed, with dozens more injured, and that the explosion appeared to be caused by a bomb.
Rebel group Ahrar al-Sham said members observing the convoy to ensure its safe passage were among those killed in the bombing.
But pro-government media and the opposition exchanged accusations, each pointing to foreign interference or conspiracies undermining the deal. People walked outside the buses, surveying the damage as well as bodies lying on the roadway and a grass median. Before midnight Saturday, 100 of some 120 buses from both sides had already arrived.
The United Nations has demanded that those responsible for the bombing be brought to justice.
The blast, carried out by a suicide bomber driving a booby-trapped potato truck, rocked the rebel-held Rashideen area in the countryside of Aleppo province, where buses carrying 5,000 pro-government Shiite people were waiting the reactivation of a deal created to secure their transportation to government-controlled area in Aleppo. According to AFP news agency, up to 5000 government evacuees and 2200 from rebel towns had been stranded in transit on Sunday.
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Foah and Kefraya, most of whose residents are Shia Muslims, have been encircled by rebels and al Qaeda-linked Sunni Muslim jihadists since March 2015. More than 50 buses and 20 ambulances carrying 5,000 Foua and Kfarya residents entered the government-held city of Aleppo, Syrian state TV said, with some of them later reaching a shelter in the village of Jibreen to the south.
Residents of Madaya and Zabadani, formerly summer resorts, joined the 2011 uprising against President Bashar al-Assad. The blast puts the four-town evacuation deal, brokered partly by rebel-backer Qatar and ally Iran, in doubt. Residents of al-Foua and Kfraya, besieged by the rebels, have lived under a steady hail of rockets and mortars for years, but were supplied with food and medicine through military airdrops. He said the area was walled off from all sides and there were no restrooms.
Maysa, a 30-year-old evacuee from Kafraya, said she was sitting in one of the buses with her six-month-old son, Hadi, and 10-year-old daughter, Narjis, when the blast shook the parked convoy.
The others were aid workers and rebels guarding the convoy, it said.
The rebels and residents of Madaya near Damascus were waiting at the government-held Ramousah bus garage, a few miles away.
The agreement proposes the transport of thousands of people out from both opposition and government-held towns under siege.