A vehicle filled with explosives hit the convoy on Saturday.
The death toll from a bomb blast has reached 126 people, while many more were injured in the attack, the Syrian Observatory for Human Rights monitoring group said on Sunday.
The blast caused buses to shatter and set cars on fire, leaving a trail of bodies.
Syrian rescue workers the Civil Defence said that they had carted away at least 100 bodies from the site of the blast, which hit buses carrying Shi'ite residents as they waited to cross from rebel into government territory in an evacuation deal between warring sides.
The UK-based Observatory said the number of deaths was expected to rise.
There has been no immediate claim of responsibility for the attack, while the United Nations has demanded that those responsible be brought to justice.
UN Secretary General Antonio Guterres condemned the bombing and called on all parties "to ensure the safety and security of those waiting to be evacuated".
Those killed were mostly residents of the villages of al-Foua and Kefraya in Idlib province, but included rebel fighters guarding the convoy, the Observatory added.
The convoy was carrying at least 5,000 people including civilians and several hundred pro-government fighters, who were granted safe passage out of the two Shi'ite villages which are besieged by rebels.