At least 18 people were killed and tens more injured, including civilians, in an explosion in Syria's northwestern city of Idlib, a war monitor reported on Sunday.
The Syrian Observatory for Human Rights said the explosion targeted the headquarters of a minor rebel faction in Idlib. It was not immediately clear what the nature of the attack was.
Ambulances and rescue teams rushed to the explosion site and efforts are under way to pull out the bodies and rescue the injured trapped under the rubble of the targeted building and neighboring houses, the monitor added.
Idlib province is a stronghold of rebels in Syria and is situated on the border with Turkey, one of the main backers of rebellion against President Bashar al-Assad.
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 main rebel force in Idlib is Tahrir al-Sham, spearheaded by the former al-Qaeda affiliate in Syria previously called Nusra Front.
The Syrian army lost Idlib to insurgents when the provincial capital fell in 2015. It became the only province fully under opposition control.
At least 18 killed in explosion in Syria's Idlib