Syrian warplanes launched air strikes on rebel enclaves across the country on Thursday, with at least 10 people killed in one raid on flashpoint town Qusayr near the Lebanon border, a watchdog said.

Areas of Damascus province, the northwestern province of Idlib, rebel-controlled Raqa in the north, Hasakeh in the northeast and Daraa in the south were also targeted, the Syrian Observatory for Human Rights said.

"The number killed in ongoing aerial bombardment of the town of Qusayr has risen to 10 men," it added.

Qusayr has in the past week become the focal point of Syria's spiralling war, and the opposition and a rights group say Lebanese Shiite movement Hezbollah has joined regular and militia fighters loyal to President Bashar al-Assad to try to crush the insurgency there.

Regime helicopters also strafed several targets in the Eastern Ghouta area, east of Damascus, a rebel stronghold and the scene of intense battles in past months.

Warplanes also struck Moadamiyet al-Sham and Daraya, southwest of the capital, where the army has battled rebels for several months.

Insurgents have used areas east and southwest of Damascus as rear bases and as gateways into the capital.

And in Hama in central Syria, fresh fighting broke out after insurgents took an army position in a significant move since the army considers the traditional opposition city to be a secure area.

The Observatory also reported fierce clashes pitting troops against rebels in the flashpoint Barzeh district of northern Damascus, which rebels have infiltrated from the east.

Elsewhere, warplanes struck Maaret al-Numan in Idlib province, as well as several villages in the Hasakeh and Daraa countryside.

In Raqa province, fighter jets struck an area near a sugar factory, and "fierce clashes pitting troops against rebels raged near army Base 17" near the provincial capital.

Rebels have had the base under siege for several weeks.

Although rebels have controlled the provincial capital Raqa since March, troops holed up in the base have managed to hold out.

According to a preliminary toll distributed by the Observatory, at least 49 people were killed in violence across Syria on Thursday.