An explosion in Syria’s northwestern city of Idlib killed at least 23 people and injured tens more, including civilians, a war monitor reported on Sunday.

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 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 neighbouring 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.