At least 43 people were killed on Saturday when a suicide bomber struck a convoy of buses carrying civilians out of two Syrian towns where they had been besieged for more than two years.
Residents of Fuaa and Kafrya had been surrounded by rebels forces since March 2015 and were finally being evacuated to regime-controlled areas under a deal reached between the Syrian government and opposition.
But as they were leaving rebel-held territory a van supposedly carrying medical supplies pulled level with their buses and exploded, according to the Syrian Observatory for Human Rights.
Footage from the scene showed bodies piled on the side of the road next to the burnt-out vehicles. Their suitcases and belongings could still be seen inside the buses that were meant to carry them to safety.
It was not immediately clear who was responsible for the attack. Syrian state television blamed the opposition, pointing out that the civilians of Fuaa and Kafrya had remained loyal to the regime during the siege and that rebel groups regularly use suicide bomber.
Opposition activists countered that the regime itself might have carried out the attack on its own civilians to deflect attention from its chemical weapons attack in Khan Sheikhoun last week. They said rebel fighters had also been killed in the attack.
The bombing took place at Rashideen, a rebel area near the city of Aleppo, which has been fully under regime control since December last year. The massive blast appeared to have destroyed at least four buses and several nearby cars.
The attack threatened to unravel the delicate deal struck between the warring sides to evacuate 30,000 people from four towns that had languished under siege for years.
Under the terms of the deal, people from the regime-controlled towns of Fuaa and Kafrya would be evacuated at the same time as residents from the rebel-held towns of Madaya and Zabadani.
Residents from Madaya, which has been wracked by starvation during the siege, said they feared regime forces might carry out attacks against them in retribution for the attack against loyalist civilians.
“We’re worried about the reaction of the regime army,” Dr Mohammad Darwish, a Madaya resident, told The Telegraph from a bus where he and others were waiting in a regime area. “We’re asking all humanitarian organisations to protect us.”
He said the buses appeared to be moving ahead towards rebel-held areas but the situation remained confused. Monitors said the evacuations also seemed to be continuing despite the attack.
Even before the bombing, the complex choreography of the evacuations had stalled and thousands of civilians spent the night sleeping on buses parked in an Aleppo depot and waiting anxiously for news.