The Syrian Civil Defense in Aleppo province, also known as the White Helmets, says its volunteers were able to remove at least 100 bodies from the scene of the blast.
"The suicide bomber was driving a van supposedly carrying aid supplies and detonated near the buses", the monitoring group said.
The current death toll has been reported by the Syrian Observatory for Human Rights and Syrian state TV.
"A very powerful blast" killed "many women and children" inside the buses, an eyewitness said while filming the video.
Local sources, speaking on condition of anonymity, said a auto rigged with explosives was detonated by remote control in Salma village, which lies northeast of the provincial capital city of Latakia.
Pro-regime media said a suicide attacker had detonated a vehicle bomb near the convoy.
Pictures shared in the aftermath of the explosion show bodies lying on the ground and fires belching out thick black plumes of smoke. The major blast hit a bus depot in a rebel-controlled area where residents of pro-government towns have arrived late Friday.
Buses carry Syrians as they leave on Friday from Madaya, an opposition-held town near Damascus, Syria.
Some 30,000 besieged people would be taken out but, according to AFP news agency, up to 5,000 government evacuees and 2,200 from rebel towns are now stranded.
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Almost 40 people have lost their lives and several others sustained injuries when a powerful bomb explosion struck near several buses carrying people from two Shia-majority villages in Syria's northwestern province of Idlib, as they were waiting to enter the city of Aleppo.
Residents of Al Fu'ah and Kefraya, in militant-controlled Idlib province, were to be transferred to the outskirts of Aleppo, which was recaptured by President Bashar Al-Assad's government last December.
The predominantly Shiite towns of Foua and Kfraya have remained loyal to the Syrian government while surrounding Idlib province has come under hard-line Sunni rebel rule.
The Observatory said the Fuaa and Kafraya buses were expected to head to government-held Aleppo city, while the Madaya evacuees were to head to Idlib.
"There's no drinking water or nourishment", Madaya inhabitant Ahmed, 24, revealed to Reuters news office prior, talking from the transport carport in Aleppo where he and others had been apparently holding up since Friday night.
"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.
No one knows the exact number of the victims yet, the eyewitness said, claiming that "there are more dead people inside the buses".
Under the agreement, 5,000 people were set to receive safe transfer from government-held towns that are surrounded by rebels and 2,000 left rebel-held towns that are surrounded by government forces.
Syria's population is mostly Sunni. Assad is from the Alawite religious minority, often considered an offshoot of Shi'ite Islam.