Regime warplanes launched air raids Thursday on a military airbase in northwest Syria to try to dislodge rebels who have seized more than half of the compound amid fierce clashes on the ground, a watchdog said.

The strikes on Taftanaz military airport came after the hardline Ahrar al-Sham and Al-Nusra Front battalions stormed it on Wednesday following a protracted siege, the Syrian Observatory for Human Rights said.

Fighting continued inside the airbase on Thursday near the main buildings as warplanes and helicopters bombed the airport and surrounding areas, the watchdog said.

The insurgents had already seized a weapons depot and captured 13 troops, including an officer, the Observatory said, adding that 11 militiamen loyal to the regime of Bashar al-Assad were reportedly taken prisoner.

According to the watchdog, the rebels also seized 16 to 20 aircraft, but they had been damaged during the clashes or disabled and were not airworthy.

The Syrian Revolution General Commission, a grassroots network of activists on the ground, reported that helicopters were dropping explosive barrel-bombs on the town of Taftanaz.

Near Damascus, loyalist troops carried out air raids on the Eastern Ghuta region and the town of Maliha on the eastern outskirts of the capital, as clashes broke out in the town of Sayyida Zeinab to the south.

The army has for months been trying to regain total control of Damascus and its environs, and battles have raged outside the capital where insurgents have set up rear bases.

The regime has frequently claimed to be waging a "final" crackdown on the rebellion in Damascus province, but such announcements have proved false.

On Thursday, the pro-regime newspaper Al-Watan reported that the army "continued to progress on all axes of Damascus, carrying out intensive military operations on all fronts despite the difficult weather conditions."

It quoted a military source as saying that "the backbone of the terrorists was crushed long ago. It has been observed that their attacks are done in small numbers and they need days to collect themselves before an attack on any facility."

A Syrian military source told AFP on Wednesday that about 4,500 fighters were encamped on the capital's outskirts, among them the Islamist Al-Nusra Front.

The source estimated that 150 fighters were still holding out in the embattled southwest town of Daraya, to where the army has repeatedly dispatched reinforcements in past weeks in an attempt to seize full control.

On Wednesday, 57 people were killed in violence across Syria, according to the Obervatory, which relies on a network of activists and medics on the ground.