Activists say Syrian government forces have killed at least 20 civilians while targeting rebels in the northwest who have been advancing in recent weeks.

The airstrikes and shelling in Idlib province overnight targeted rebels in areas north and south of the town of Jisr al-Shughour, which recently fell to the insurgents, and in and around the town of Saraqeb.

The Britain-based Syrian Observatory for Human Rights said Thursday at least 21 civilians, including five children, were killed.

Activist groups on Wednesday said government helicopters had dropped explosives-filled barrels containing chlorine on Saraqeb. The reports could not be independently verified.

The insurgents have been advancing in Idlib province in recent weeks, capturing the provincial capital as well as Jisr al-Shughour and a military base.