A huge car bomb has blasted a convoy of coaches carrying evacuees from besieged 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.
There are fears of revenge attacks on a convoy of evacuees from rebel-held towns, being moved under a deal.
But reports suggest both convoys have resumed their journey or will do soon.
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.
About 20,000 besieged people would be taken out in all. According to AFP news agency, up to 5,000 government evacuees and 2,200 from rebel towns had been stranded in transit on Sunday.
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.
What do we know of the bombing?
The bomb reportedly went off at Rashidin, west of government-held 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.
"There are dead people everywhere. You can see tens of burnt out cars, bodies everywhere," said an unnamed eyewitness, describing the carnage for Syria's Qasioun news agency.
"Emergency staff and opposition factions are evacuating the wounded and the martyrs [the dead]."
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, accusing the government of trying to bring out more loyalist fighters than agreed.
Where does this leave the rebel evacuees?
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."
Madaya resident Ahmed, 24, told Reuters news agency earlier that evacuees had been waiting without drinking water or food at a bus garage in Aleppo since Friday night.
"The bus garage is small so there's not much space to move around," he said. "We're sad and angry about what has happened."
The Madaya and Zabadani evacuees are meant to be transferred to rebel-held territory in Idlib province.
Why are the evacuations needed?
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 Muslim, 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.