The UK-based Syrian Observatory for Human Rights said the toll was expected to rise due to the large number of people who suffered serious injuries.
A video posted by activists online showed the aftermath of the blast, with at least one car ablaze and the ground around it covered in smoking embers.
A second video showed residents carrying bodies on makeshift stretchers and extensive damage to buildings around the blast site.
“The area is controlled by rebels; government forces were pushed out months ago. There are no claims of responsibility,” said Al Jazeera’s Zeina Khodr, reporting from Beirut, the capital of neighbouring Lebanon. “There’s still a lot of insecurity in the north, even though most of that area is held by the rebels.”
The car bombing came hours after the International Committe of the Red Cross reported the abduction of six of its staff along with an employee of the Syrian Arab Red Crescent.
The ICRC vowed on Monday that it would not stop work there, despite the weekend kidnapping.
“We are completely committed to supporting the Syrian population in this difficult moment,” ICRC spokesman Ewan Watson told Swiss public radio.
“We don’t have any intention of stopping our activities in Syria, but of course this situation makes us reflect and take a close look at our operations because in the end, we will not be able to work and help the Syrian population without having security for our personnel,” he said.
“We are worried that these types of incidents will prevent us from having as large of an access in the future and from carrying out our humanitarian work,” he added.
The workers were abducted by gunmen on Sunday as their convoy drove from the Idlib province back to Damascus.
Watson did not identify the nationalities of the staff kidnapped, saying only that most of them were Syrian.
No group has yet claimed responsibility for the abduction.
Kidnapping has become an increasing problem in Syria, with journalists and aid workers frequently targeted in rebel-held parts of the country, largely in the north.
Last month a German aid worker held for almost four months escaped his kidnappers in Idlib, just like his two colleagues who managed to flee in July.