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More than 100 people were killed in a car bomb blast in northern Syria on Saturday, when an explosion hit through the buses which were evacuating residents from two towns besieged by rebels, reports state.

The Syrian Observatory for Human Rights said that the blast was targeting buses which were carrying residents who were evacuated from the northern towns of Fuaa and Kafraya. The evacuation was reached under a deal between the Bashar al-Assad-led Syrian regime and rebels. The Syrian state television showed footage of burnt vehicles packed with people's luggage and bodies strewn across the ground.

Syria's six-year-long civil war between President Bashar al-Assad's regime and opposition rebels has left more than 320,000 people dead since 2011.

"The suicide bomber was driving a van supposedly carrying aid supplies and detonated near the buses," the monitoring group said, according to AFP reports. The human rights group also said that although most of the people who died in the explosion were evacuees, but the blast also killed several rebels who were guarding the buses in Rashidin, west of Aleppo.

The blast left thousands more evacuees stranded on the road over a disagreement on number of rebels allowed to leave other two towns which were included in the deal. The Observatory said that the evacuation process was resumed soon after the blast.

Reports state that several people were wounded in the incident, including women and children who were taken to a hospital in the government-held part of Aleppo for treatment.

The bombing was condemned by the rebel group Ahrar al-Sham, which said that they were securing the civilians at the time.

"We reject any accusations levelled at opposition for this heinous crime," a senior official tweeted. "Our role was to secure civilians not kill them."

More than 5,000 people who had been living under siege for more than two years, left the two towns on Friday. Reports state that around 2,200 people were also evacuated from rebel-held Madaya and Zabadani.