BEIRUT: Syrian rebels, some from Islamist units, fired machine guns and mortars at helicopters grounded at a northern military air base near the main Aleppo-Damascus highway on Wednesday, a monitoring group said.

The al-Nusra Front (allegedly linked with Al Qaeda), 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 (SOHR) 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 considerable number 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 Wednesday'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.

Syria's civil war is the longest and deadliest conflict to emerge from uprisings that began sweeping the Arab world in 2011 and has developed a significant sectarian element.

Rebels, mostly from the Sunni Muslim majority, confront Assad's army and security forces, dominated by his Shia-derived Alawite sect, which, along with some other minorities, fears revenge if he falls.

UN-led diplomatic peace efforts have stumbled. Western and many Sunni Arab states demand Assad's immediate removal, an idea resisted by Russia, China and Syria's ally Iran.

The rebels say they will not negotiate unless Assad, who has vowed to fight to the death, leaves power.

More than 110 people, including at least 31 of Assad's soldiers and militiamen, were killed in Syria on the first day of 2013, according to the Observatory, which tracks the conflict from Britain using a network of contacts inside the country.