Under a complex humanitarian deal in Syria, 5,000 people were offered safe passage from the government-held towns of Foua and Kefriya, which are surrounded by rebels, and 2,000 left the rebel-held towns of Madaya and Zabadani near Damascus, which were besieged by government forces.
The exchange began early on Saturday, but both groups became stranded outside Aleppo, after the two sides started wrangling over the number of fighters to be evacuated.
Then, a van used to carry humanitarian supplies pulled up to some of the lead buses carrying the evacuees destined for government areas. But it was a suicide bomber.
At least 129 people died and the landscape was littered with body parts and the remains of household goods.
A senior rebel official said about 20 fighters guarding the buses had been killed, and it was feared that some of the casualties were aid workers.
Residents of rebel-held towns waiting in buses in government territory said they were terrified they could face reprisal attacks.
They appealed to international organizations including the UN for security guarantees, asking them to “provide the necessary protection to reach our final destinations”.
UN secretary-general Antonio Guterres condemned the attack and called on all parties “to ensure the safety and security of those waiting to be evacuated.”
No group has claimed responsibility for the attack and eventually – and to everyone’s relief – replacement buses arrived, the wounded were taken to hospitals and the evacuations resumed.
The population shift is an attempt to alleviate the hardships of residents of towns under siege by both rebel and government forces.
Condemning the bombing, UN aid chief Stephen O’Brien said: “The perpetrators of such a monstrous and cowardly attack displayed a shameless disregard for human life.”
Pope Francis on Sunday also urged an end to the war in Syria as he presided over the traditional Easter mass in Rome.