The al Qaeda-linked al-Nusra Front, Ahrar al-Sham Brigade and other units operating in Syria's northwestern province of Idlib were attacking the Afis military airport near Taftanaz, the pro-opposition Syrian Observatory for Human Rights said.

There was no immediate account of the fighting around the air base from Syrian state media.

Insurgents trying to topple President Bashar al-Assad see his air power as their main threat. They hold swathes of eastern and northern provinces, as well as a crescent of suburbs around the capital, Damascus, but have been unable to protect rebel-held territory from relentless attack by helicopters and jets.

In recent months, rebel units have besieged several military installations, especially along Syria's main north-south artery from Aleppo, its most populous city, to Damascus.

The Observatory's director, Rami Abdelrahman, said today's attack was the latest of several attempts to capture the base. A satellite image of the airport shows more than 40 helicopter landing pads, a runway and aircraft hangars.

An estimated 45,000 people have been killed in the Syrian conflict, which began in March 2011 with peaceful protests against four decades of Assad family rule but turned into an armed revolt after months of government repression.

In Damascus, Assad's forces fired artillery and mortars at the eastern districts of Douma, Harasta, Irbin and Zamlaka, where rebels have a foothold, activists living there said.