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The nature of the attack wasn't 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 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.
Idlib province, a stronghold of rebels in Syria, is on the border with Turkey, one of the main backers of rebellion against President Bashar al-Assad.