A huge car bomb has blasted a convoy of coaches carrying evacuees from government-held towns in Syria, killing at least 39 people, state media report.
It shattered coaches and set cars on fire, leaving a trail of bodies including children, as the convoy waited in rebel territory at Rashidin.
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 was meant to relieve suffering in areas under siege.
Some 30,000 besieged people would be taken out of two rebel-held towns and two under government control 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 pro-government north-western towns of Foah and Kefraya, and the rebel-held towns of Madaya and Zabadani near Damascus, as “catastrophic”.
More than 64,000 civilians are “trapped in a cycle of daily violence and deprivation”, it said.
The bomb reportedly went off 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 the 24Aleppo news site quoted its own source as saying 60 civilians had been killed.
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 coaches.
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.
The Syrian Arab Red Crescent was distributing food and water to the waiting passengers, who include 3,700 civilians, the agency adds.
Rebels say Damascus breached the terms of the deal brokered by Iran and Qatar, accusing the government of trying to bring out more loyalist fighters than agreed.
According to an 24Aleppo tweet, a special unit of the Russian army surrounded the convoy from the rebel-held towns after the bombing and closed the road there to “prevent any reaction”.
Evacuees from Madaya called on international organisations to protect them from any possible retaliation, saying they condemned the attack on the other convoy.
“Everyone here is on a tether following the Rashidin bombing,” they said in a statement.
“We are holding the guarantors of the Four Towns deal responsible for our safety as the Shia towns and regime troops around us are on the highest alert.”
“There’s no drinking water or food,” Madaya resident Ahmed, 24, told Reuters news agency earlier, speaking from the bus garage in Aleppo where he and others had been reportedly waiting since Friday night.
“The bus garage is small so there’s not much space to move around. We’re sad and angry about what has happened.”
The evacuees are meant to be transferred to rebel-held territory in Idlib province.
Many people are reported to have died as a result of shortages of food or medicine in the four towns.
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.
Madaya and Zabadani, which are predominantly Sunni, have been besieged since June 2015 by the Syrian army and fighters from Lebanon’s Shia Muslim Hezbollah movement.
A previous attempt at mutual evacuations failed in December when rebels burnt coaches due to be sent to the towns.