Smoke rises from a burnt car at the site of a truck blast in Azaz, Syria, on Sunday. Khalil Ashawi/Reuters

At least seven people were killed on Sunday in a car bomb that hit a rebel-held city in northwestern Syria, a war monitor said.

More than 20 others were wounded in the explosion in Azaz, located in the heart of a Turkish zone of influence in Aleppo province, said the Syrian Observatory for Human Rights.

The Britain-based monitor said the car bomb targeted a bustling area located near several restaurants, leading to severe material damage.

It was not clear who was behind the attack or if the casualties were all civilians.

Turkey launched Operation Euphrates Shield in 2016 and seized more than 2,000 square kilometres of northern Syria including Azaz, clearing the area of Islamic State jihadists while preventing any Kurdish advance in the region.

Ankara keeps Turkish troops and intelligence forces in the area, and still backs Syrian rebels serving as local police officers.

Rebel-held parts of northern Syria are regularly hit by car bombs and explosions.