Out of 126 people killed in the bus explosion outside Aleppo on Saturday, 15 April, at least 26 were children, according to human rights groups, and the death toll expected to rise further.
The UK-based Syrian Observatory for Human Rights stated that over 100 evacuees from government-held towns had died along with aid workers and rebel militia.
A spokesman for the leading rebel faction Ahrar al-Sham said one-third of the casualties were rebels accompanying the convoy.
A vehicle packed with explosives targeted a convoy near Aleppo, which destroyed buses and set cars alight, with horrific scenes of body parts strewn across the road.
Pope Francis, in his Easter Sunday address, called the massacre a "vile attack on fleeing refugees".
The pontiff added: "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."
The bomb blast happened in Rashidin, to the west of government-held Aleppo. A suicide bomber drove a vehicle stocked with food and started distributing crisps to children before detonating his bomb, Lina Sinjab, the BBC's Middle East correspondent said.
Rami Abdul Rahman, director of the Syrian Observatory for Human Rights, said in a televised interview that the bomber claimed he was carrying food supplies and blew himself up using an improvised explosive device in a vehicle.
"It appears that the explosion happened at the front of the convoy, which is about 70-buses long. Apparently, it happened in an area where the sick and the injured were either being transferred or swapped," said Al Jazeera's Hoda Abdel-Hamid.
No organisation has claimed responsibility for the attack, which pro-Damascus media said was carried out by a suicide car bomber. Groups fighting under the banner of the Free Syrian Army condemned it as a "treacherous terrorist attack".
The evacuation continued on Sunday (16 April), according to Al Ahram. Over 3,000 Syrians will be moved as a part of a population transfer that stalled due to the deadly blast. Critics of the so-called "four towns" deal, which could see a total of 30,000 people being moved, say it amounts to forced displacement across sectarian lines.