More than 40 people have been killed and at least 100 others injured in Syrian government air strikes on a residential area of a rebel-held town, international watchdog Human Rights Watch said today.

The strikes on the town of Azaz in northern Syria levelled most of a poor neighbourhood and sent panicked civilians fleeing for cover.

So many people were wounded that the local hospital locked its doors, directing residents to drive to the nearby Turkish border so the injured could be treated on the other side.

Reporters from The Associated Press saw nine bodies in the bombings' immediate aftermath, including a baby.

Human Rights Watch, which investigated the site of the bombing two hours after the attack, put the number at more than 40.

"This horrific attack killed and wounded scores of civilians and destroyed a whole residential block," said Anna Neistat, the group's acting emergencies director. "Yet again, Syrian government forces attacked with callous disregard for civilian life."

HRW said two opposition Free Syrian Army facilities in the vicinity of the attack might have been the targets of the Syrian aircraft.

One was the headquarters of the local Free Syrian Army brigade two streets away from the block that was hit. The other was a detention facility where the Free Syrian Army held "security detainees" - government military personnel and members of pro-government shabiha militia. Neither of these facilities was damaged in the attack.

The bombing of Azaz, some 50km north of Aleppo, shattered the sense of control rebels have sought to project since they took the area from President Bashar Assad's army last month.

Azaz is also the town where rebels have been holding 11 Lebanese Shiites they captured in May. Yesterday, Lebanese media reported conflicting reports on their fate, but it was unclear whether they had been affected by the bombing.

In recent months, rebels have pushed the Syrian army from a number of towns in a swath of territory south of the Turkish border and north of Aleppo, Syria's largest city. About a dozen destroyed tanks and army vehicles are scattered around Azaz, left over from those battles.

As the Assad regime's grip on the ground slips, however, it is increasingly targeting rebel areas with attack helicopters and fighter jets - weapons the rebels can't challenge.