A huge car bomb has blasted a convoy of coaches carrying evacuees from government-held towns in Syria, killing at least 39 people.
It shattered coaches and set cars on fire, leaving a trail of bodies including children, as the convoy waited in rebel territory near Aleppo.
Russian troops have reportedly moved to shield rebel evacuees from retaliation.
Thousands of evacuees from both sides of Syria’s civil war have been stuck in hostile territory since Friday.
The “Four Towns” deal brokered by Iran and Qatar was meant to relieve suffering in besieged towns – Foah and Kefraya in the north-west which are under government control, and rebel-held Madaya and Zabadani near Damascus.
Some 30,000 besieged people would be taken out but, according to AFP news agency, up to 5,000 government evacuees and 2,200 from rebel towns are now stranded.
Last month, the UN described the situation in the besieged towns as “catastrophic”. More than 64,000 civilians are “trapped in a cycle of daily violence and deprivation”, it said.
The bomb reportedly went off at Rashidin, west of Aleppo, around 15:30 local time (12:30 GMT) at the checkpoint where the handover was due to take place.
Syrian state media reported 39 deaths while other sources spoke of between 43 and 60 deaths. Hundreds of people are said to have been injured.
A suicide bomber driving a van supposedly carrying aid supplies blew it up near the coaches, the UK-based Syrian Observatory for Human Rights reports.
Images from the scene show bodies lying on the ground outside blackened and devastated vehicles.
The complex choreography of this exchange has been attempted before on a smaller scale, reports Sebastian Usher, the BBC’s Arab affairs editor. There must now be concern over whether it can continue at all, he adds.
An AFP correspondent west of Aleppo, speaking before the explosion, said the coaches carrying government evacuees had not moved in 30 hours.