A large blast has killed at least 126 people, including 68 children, in an attack near buses carrying Syrians evacuated from two besieged government-held towns, according to a monitoring group.

The explosion on Saturday in Rashidin, west of Aleppo, targeted residents who were evacuated from the rebel-besieged towns of Foua and Kefraya in Idlib province under a deal reached between the Syrian government and rebels.

The UK-based Syrian Observatory for Human Rights (SOHR) on Sunday said at least 126 people were killed in the blast, raising a previous death toll of 112.

The Syrian Civil Defence, a volunteer rescue group that operates in rebel-held parts of Syria, had said on Saturday that at least 100 people were killed, while a report on Syrian state TV had given a death toll of 39.

While there was no confirmation as to what caused the blast, state media said a "suicide bomber" allegedly used a van meant for carrying aid supplies to enter the area. The SOHR said the explosion came from a vehicle-born improvised explosive device.

Al Jazeera's Adham Abul Hussam, reporting from the scene of the attack, said dozens of bodies littered the ground and ambulances were packed with victims.

"The civil defence teams are recovering the bodies and searching for survivors. Many of the buses were totally destroyed," he said.

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Pictures posted on state media showed what appeared to be the aftermath of the explosion, with burned bodies and fires belching out thick black smoke.

Buses were blackened by the blast with their windows blown out.

"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," Al Jazeera's Hoda Abdel-Hamid said, reporting from Antakya in Turkey.

"We do understand that the casualties go into the dozens … among them women and children and some of the rebel fighters that were there to secure that convoy."

Ahrar al-Sham, a key rebel group in Syria's north, condemned the "cowardly" attack, saying that many of its members were killed in the blast. The group said it was willing to cooperate with an international probe to determine the culprits.

The attack took place as thousands of evacuees from the besieged government-held towns of Foua and Kefraya waited to continue their journey to regime-controlled Aleppo, the coastal province of Latakia, or the capital, Damascus.

More than 5,000 people who had lived under the crippling siege for more than two years left the two towns, along with 2,200 evacuated from rebel-held Madaya and Zabadani, on Friday.

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They were headed for regime or rebel-held areas via the government-held second city of Aleppo.

Thousands of evacuees from Foua and Kefraya were stuck on the road in Rashidin when the blast struck.

The evacuation, brokered by government ally Iran and rebel-backer Qatar, is set to see more than 30,000 people evacuated over 60 days.

The deal had stipulated that in the first stage 8,000 people, including 2,000 pro-government fighters, leave the two towns. But only 5,000 left, including 1,300 fighters, the Observatory said.

Evacuees were left stranded as differences emerged over the number of loyalist fighters leaving, a rebel source said, refusing to elaborate as "negotiations are under way".

Thousands of evacuees from Madaya and Zabadani were also stuck in government-controlled Ramousa, south of Aleppo.They were due to be transferred to rebel-held Idlib.

Mohamed Darwish, a doctor from Madaya, told Al Jazeera by phone that there was a growing fear among civilians aboard buses in Ramousa.

"There's a lot of fear on our bus, especially because the regime soldiers are here and there is a crowd all around us," he said.

"We hope the UN, Turkey, Qatar, Iran, all of those who supported this deal, will look after the civilians and ensure that they all arrive safely."

But hours after the explosion, the transfer resumed - as dozens of buses, starting with the wounded, left to their respective destinations. Before midnight on Saturday, 100 of some 120 buses from both sides had already arrived to their destinations.

The deal to evacuate the towns was the latest in a string of such agreements, touted by the government as the best way to end the fighting. Rebels say they have been forced out by siege and bombardment.

The government has retaken several key rebel strongholds including eastern Aleppo since Russia's military intervention in September 2015.

Additional reporting by Dylan Collins: @Collinsdyl