Photos that were too graphic to publish showed a huge fire raging next to bodies scattered on the ground next to charred buses, including those of children.
The residents, alongside hundreds of pro-government fighters, had left the two rebel-besieged villages in northwest Idlib province under a deal where in exchange, hundreds of Sunni insurgents and their families moved out of a government-besieged area near Damascus.
Some 30,000 people are being relocated from 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.
State media says the incident was caused by a "suicide bomber" who reportedly used a van meant for carrying supplies to gain entrance to the area.
"I know nothing of my family".
The rebel group Ahrar al-Sham tweeted that some of its members died in the blast.
Those killed included civilians as well as rebel fighters. The group said it was investigating to find out who was responsible.
The Syrian Civil Defence in Aleppo province, also known as the White Helmets, said its volunteers were able to remove at least 100 bodies from the scene of the blast. A war monitor puts the death toll at 24 in the area controlled by opposition fighters.
According to Abdul Hakim Baghdadi, an interlocutor who helped the government negotiate the evacuations, 140 were killed in the attack. It was not immediately clear if al-Hakim was the same French national US officials said was killed in an airstrike in Syria in November 2016.
A suicide auto bomb attack on buses carrying Syrians evacuated from two besieged government-held towns killed 24 people on Saturday, as US-backed fighters advanced in their push towards the Islamic State group's Raqa stronghold.
U.N. Secretary-General Antonio Guterres condemned the attack Saturday in a statement from his spokesman Stephane Dujarric, and called on all parties "to ensure the safety and security of those waiting to be evacuated".
Government: 36 Islamic State fighters killed by massive bomb
He added, "We have USA forces at the site, and we see no evidence of civilian casualties nor have there been reports ". About 36 suspected militants were killed, according to Afghan officials, but IS has denied suffering any casualties.
Madaya and Zabadani, once summer resorts to Damascus, have been shattered under the cruelty of a government siege.
Foua and Kfraya, besieged by the rebels, lived under a steady hail of rockets and mortars.
The explosion came as frustration was already mounting over the stalling evacuation process.
"The people are restless and the situation is disastrous", said Ahmed Afandar, a resident evacuated from his the opposition area near Madaya.
He said the area was walled off from all sides and there were no toilets.
But a delay in the agreement had left all those evacuated stuck at two transit points on Aleppo's outskirts since late on Friday.
The deal had stipulated that in the first stage 8,000 people, including 2,000 loyalist fighters, leave the two towns but in the event just 5,000, including 1,300 fighters left, the Britain-based Observatory said.
Thousands of evacuees from Madaya and Zabadani were also stuck in regime-controlled Ramusa, south of Aleppo.
Syrian state TV blamed the rebels for obstructing the deal.
An opposition representative, Ali Diab, told the pan-Arab Al-Arabiya TV that fewer armed men than agreed to were evacuated from the pro-government areas, violating the terms of the agreement.