The explosion hit the al-Rashideen area, a rebel-controlled district outside Aleppo city where evacuation buses carrying almost 5,000 people from the northern rebel-besieged villages of Foua and Kfraya were stuck.
More than 64,000 regular people are "caught in a cycle of day by day brutality and hardship", it said.
"I just couldn't bear it".
People waiting in the Ramousah garage heard the blast, and said they feared revenge attacks by pro-government forces. Critics say the deal amounts to "demographic engineering".
The Syrian Red Crescent, who was delivering food and water, said there were 3,700 civilians among the bus passengers.
Terrorists reportedly loaded the auto in which they hid the bomb with children's food supplies to sneak it into the area, SANA reported, citing its correspondent.
The explosion was caused by a auto bomb, according to Syrian TV and the opposition Britain-based Syrian Observatory for Human Rights, who put the death toll at 24 and said it would likely rise.
The Observatory said the delay was caused by the fact that rebels from Zabadani, another town near Damascus included in the deal, had not yet been granted safe passage out.
According to Abdul Hakim Baghdadi, an interlocutor who helped negotiate the evacuations, government conscripts will stay and defend the towns.
The evacuation of Fua and Kefraya residents is a part of a deal between the Syrian government and militants. Residents from the two villages had been evacuated Friday, along with more than 2,000 from Madaya, an opposition-held town outside of Damascus besieged by government forces.
Residents of Foua and Kefraya were agreed to be transferred to the outskirts of Aleppo City, the coastal province of Latakia or Damascus, while the gunmen and their families would leave for Idlib City.
Moscow, Tehran, Damascus seek on-site probe into Syria gassing
It was Assad's first since the alleged April 4 chemical weapons attack prompted a USA cruise missile strike on a Syrian air base. Sergey Lavroy, Russian Foreign Minister, hosted a trilateral meeting with Iran and Syria expressing his concerns on the matter.
Civilians from Madaya and Zabadani will reportedly be allowed to remain if they choose.
It shattered coaches and set cars on fire, leaving a trail of bodies, as the convoy waited at a marshalling point in rebel-held territory at Rashidin.
Ahrar al-Sham group is the main negotiator with the government and its allies of the widely criticized evacuation deal that transferred thousands of government and opposition supporters from four besieged areas.
But access has been limited, with food and medical shortages causing malnutrition, illness and even death among besieged residents.
The auto bombing killed at least 43 people, a lot of them from regime-held Fuaa and Kafraya, as the buses were held up in a rebel-controlled zone, according to the Syrian Observatory for Human Rights.
But in Wednesday's interview, the president told AFP that it was the rebels who were driving people from their homes.
A auto bomb in northern Syria killed dozens Saturday when it ripped through buses evacuating residents from a besieged government town.
"But of course they're going to go back to their cities after the liberation". The government blamed the rebels for the attack.
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He insisted that Organisation for the Prohibition of Chemical Weapons inspectors should visit both the air base and Khan Sheikhoun as soon as possible. "We are not moving forward or backward".
"They say still that it is not very safe, but they can not put forward convincing arguments", Lavrov said.