The blast took place near an area where buses carrying almost 5,000 people from Foua and Kfraya, villages in northern Syria was besieged by rebels, Associated Press reported. But during the last minute, the government and rebels disagreed over the number of gunmen to be evacuated, the buses were left stuck at two separate parts, and adjacent parts of the city.
A Madaya resident, speaking from the bus garage inside Aleppo, said people had been waiting there since late on Friday, and were not being allowed to leave.
Activists and residents say thousands of Syrians evacuated from their besieged towns have spent the night on buses at an exchange point as a much criticized population transfer deal stalls.
The deal to evacuate the four towns is the latest in a string of such agreements through Syria's six-year civil war.
Syria's opposition says the evacuation deals, which include areas of Aleppo and a district in the city of Homs, amount to forced displacement of Assad's opponents from Syria's main urban centres in the west of the country.
According to AFP, about 5,000 of the government evacuees and 2,200 rebel evacuees were stranded.
The deal had stipulated that in the first stage 8,000 people, including 2,000 loyalist fighters, would leave the two towns but in the event just 5,000, including 1,300 fighters left, the Britain-based monitoring group said.
Still image shows bodies lying near burnt out buses in what is said to be Aleppo's outskirts.
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Nihad Awad, national executive director of the Council on American-Islamic Relations, tweeted that he was devastated by the news. Abdus-Salaam was born into a working class family in Washington DC in 1952 and educated at public schools.
A rebel fighter stands near buses carrying people who were evacuated from villages in Aleppo after a stall in an agreement between rebels and Syria's army.
Earlier on Saturday, at the transit point where the buses from al-Foua and Kefraya were waiting, one resident said he was not yet sure where he would live.
Allison further points out that the deal has been controversial and subject to accusations of forcing demographic change. "After Aleppo I'll see what the rest of the group is doing, if there are any preparations".
The Syrian Arab Red Crescent was distributing food and water to the waiting passengers, who include 3,700 civilians, the agency adds.
"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.
Foah and Kefraya, most of whose residents are Shia Muslims, have been encircled by rebels and al-Qaeda-linked Sunni Muslim jihadists since March 2015. "We're sad and angry about what has happened", he said.
"It is an attack on people at their most vulnerable".
Syria's population is mostly Sunni.
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