BEIRUT — A car bomb in northern Syria killed more than 100 people Saturday when it ripped through buses evacuating residents from a besieged government town.

Syrian state television showed bodies strewn across the road, the charred vehicles still packed with their passengers’ possessions.

The vehicles had left the northern town of Fouaa on Friday, traveling as part of a complex population transfer brokered by regional powers over two years of negotiations.

There was no immediate claim of responsibility for the attack, and its implications for the broader deal – determining the future of some 30,000 Syrians across four towns – was unclear.

The White Helmets rescue group said its volunteers had recovered more than 100 bodies from the wreckage, and that another 55 people had been injured.

In one video from the site, a shaken reporter said the dead included civilians as well as rebel fighters from the area in which the blast had taken place. Using a cameraphone to capture the devastation, his video showed the charred bodies of two small children dangling from vehicles.

The departures Friday under the agreement marked the first stage in the population swap between rebel- and government-held areas. Overseen by Qatar and Iran, the arrangement was reached with negligible involvement by the Syrian government, underscoring how much control President Bashar al-Assad has lost in certain areas.

There already was mounting frustration when the explosion occurred: Thousands of evacuees from pro-government and opposition areas were stuck on opposite sides of the edge of Aleppo city as rebels and government bickered over the terms for evacuating fighters.

The population shift is an attempt to alleviate the hardship of residents of towns under siege by both rebel and government forces.

Fouaa and neighboring Kefraya have been reliant on government airdrops since rebel forces cut supply lines and launched frequent attacks on the towns.

Opposition officials have accused the Syrian government and its Iranian allies of using siege tactics to force demographic changes across the country, mostly along sectarian lines.

In Syria’s south, the opposition-held town of Madaya has endured a siege so tight that dozens have starved to death.

Saturday’s attacks sent shock waves through another convoy traveling under the terms of the deal. Inside those buses, the rebels, activists and doctors warned they could become “sitting ducks” in the face of retaliatory violence.

“Everyone is tense here; we feel like the attacks could start at any time,” said one man, asking that his name be withheld to protect the security of family members still in Madaya.