The Syrian Observatory for Human Rights said the explosion targeted the headquarters of a minor rebel faction in Idlib .
The nature of the attack was not immediately clear. The monitor said there were conflicting accounts, attributing the explosion that shook the Thalatheen district of the city to either a car bomb or a drone attack.
Ambulances were deployed to the explosion site and rescue teams were continuing efforts to recover bodies and the injured from the rubble of the targeted building and neighboring houses, the monitor added.
The majority of the deaths were fighters from the rebel group, in addition to seven civilians, with several people still unaccounted for, it said.
The Syrian army and its allies launched an offensive in October to recapture the provinces of Idlib and Hama, and it has since been making swift advances.
The Syrian army lost Idlib to insurgents when the provincial capital fell in 2015. It became the only province fully under opposition control.