At least 68 children were among 126 people killed in Saturday’s bomb attack on buses carrying evacuees from besieged Syrian towns, activists say.
A vehicle filled with explosives hit the convoy near Aleppo.
The UK-based Syrian Observatory for Human Rights monitoring group said at least 109 evacuees from government-held towns were killed, along with aid workers and rebel soldiers.
Many more were injured in the attack, the group said.
The explosion shattered buses and set cars on fire, leaving a trail of bodies, as the convoy waited in rebel territory near Aleppo.
In his Easter Sunday address, Pope Francis called the bombing a “vile attack on fleeing refugees”.
“May [God] sustain the efforts of those who are actively working to bring comfort and relief to the civilian population in beloved Syria, who are greatly suffering from a war that does not cease to sow horror and death,” he said.
The bomb went off at Rashidin, west of government-held Aleppo, at about 15:30 local time (12:30 GMT) at the checkpoint where the handover of evacuees was due to take place.
It happened when a vehicle loaded with food arrived and started distributing crisps, attracting many children, before exploding, the BBC’s Middle East correspondent Lina Sinjab said.
She said it was not clear how the vehicle could have reached the area without government permission.
But there is also no evidence that rebels were involved in the attack, as the government claims.
It would not be in the rebels’ interest, our correspondent says, as they were waiting for their own supporters to be evacuated from the other towns.
The planned evacuation was part of the so-called “four towns” deal, where civilians in towns under siege by both sides would be allowed to leave.
It applies to government-controlled Foah and Kefraya, where the bomb struck, as well as rebel-held Madaya and Zabadani near Damascus.
There were fears of revenge attacks on evacuees from rebel-held towns, being moved under a deal.
But the exchange later resumed, with coaches reaching safety on both sides. Evacuations were due to continue on Sunday.
Rebels had earlier accused the government of breaching the terms of the deal, accusing it of trying to bring out more loyalist fighters than agreed, along with civilians.
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.