The Syrian government said the vehicle had been supposedly carrying aid to the evacuees, while rebels claim the auto was stationary and had been parked near the stalled convoy. According to the news service, state television aired images of buses "charred" and "gutted" by the blast and bodies lying near the site of the attack.
People waiting in the Ramousah garage heard the blast, and said they feared revenge attacks by pro-government forces.
Activists and residents say thousands of Syrians evacuated from their besieged towns have spent the night on buses at an exchange point as a much criticized population transfer deal stalls. The Independent is reporting that the blast shocked a bus parked near the Rashidin area located on the outskirts of Aleppo, just as it was set to evacuate to a safer area. "All these thousands of people are stuck in less than half a kilometer".
More than 5,000 people who had lived under crippling siege for more than two years left the two towns, along with 2,200 evacuated from rebel-held Madaya and Zabadani, on Friday.
"The suicide bomber was driving a van supposedly carrying aid supplies and detonated near the buses", the monitoring group said.
Syria's opposition had said the evacuation deals, which have included areas of Aleppo and a district in the western city of Homs, amount to forced displacement of President Bashar al-Assad's opponents from Syria's main urban centres.
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At least 39 people were killed and many others were injured in an explosion that hit near buses carrying evacuees from a besieged area of government loyalists. A photo carried by al-Ikhbariya state TV showed a number of bodies strewn on the floor with a huge plume of black smoke rising in the background.
People who were evacuated from the two villages of Kefraya and Foua wait near buses, after a stall in an agreement between the opposition and Syrian regime army, at Rashideen. A Facebook page belonging to the pro-government Foua and Kfraya villages said all those in three buses were killed or are still missing while a rebel official said at least 30 opposition fighters who were guarding the evacuees were killed in the blast.
Ahmed Afandar, a resident evacuated from his hometown near Madaya, says dozens of buses carrying children, women and men are not allowed to proceed toward rebel-held Idlib as planned.
The British-based Syrian Observatory for Human Rights said the explosion appeared to have been caused by a bomb.
SDF fighters are within a few hundred metres (yards) of Tabqa and engaged in heavy fighting as IS counter-attacked, said Observatory head Rami Abdel Rahman.