At least 18 people have been killed in a car bomb blast in the country's largest rebel-held city of Idlib, according to Syrian activists and a war monitoring group.

The Britain-based Syrian Observatory for Human Rights says a car bomb was detonated on Sunday evening in Idlib, the capital of the province with the same name.

Photos and video from the activist-run Thiqa News Agency and Baladi News Agency show heavy damage along what appears to be a major avenue in the city, with several buildings damaged and vehicles overturned.

First responders, ambulances, fire brigades are seen arriving at the scene.

No group immediately claimed responsibility for the bombing. The city and the province are controlled by several rebel factions and insurgents vying for dominance, the most powerful being an al-Qaida-linked group.

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.