An air raid on a town partly held by rebels on the outskirts of Damascus on Wednesday killed at least 12 members of the same family, most of them children, a watchdog said.

Regime warplanes bombed the town of Moadamiyet al-Sham, southwest of the capital, the Syrian Observatory for Human Rights said.

Elsewhere, fierce fighting that erupted overnight in the northwest of the country raged into the day as rebels attacked regime troops stationed around an airbase causing casualties on both sides, the Britain-based Observatory said.

Outside Damascus, warplanes attacked the towns of Shebaa to the southeast and Deir Assafir to the south, where according to Human Rights Watch 11 children were killed in November when cluster bombs were dropped on a playground.

The deadly strikes came as forces loyal to President Bashar al-Assad shelled rebels in Harasta and Douma, insurgent strongholds to the northeast of the capital, and in Daraya to the southwest.

Army reinforcements have been massing for weeks in Daraya in a bid to drive rebel Free Syrian Army fighters from the town, the site of the bloodiest massacre of the conflict in which hundreds died in August.

Northeast of Damascus, the army shelled Qaboon district, the Observatory said, while in the south, residents of a Palestinian refugee camp that faced deadly air raids in December were sent fleeing once again by barrages of mortar fire.

In northwestern Syria, clashes between mostly jihadist rebel fighters and Assad's forces at Taftanaz airbase in Idlib province killed four insurgents and an unknown number of soldiers, the Observatory said.

A local resident told AFP that the army was carrying out air raids around the Taftanaz base in an attempt to repel the multi-pronged attack headed by the Islamist groups the Al-Nusra Front and Ahrar al-Sham.

The rebel assault came after authorities announced the temporary closure of the international airport in Aleppo province on Tuesday, after days of attacks there by the insurgents who hold vast swathes of territory in the north.

Fighting also broke out around the crucial Wadi Deif base, one of the last regime bastions in northwestern Syria, in a fresh jihadist-led bid to wrest control of the strategic post.

An Australian national fighting alongside rebels was among those killed, the Observatory said.

Australian citizen "Abu al-Walid al-Australi was killed ... in a rebel assault on Wadi Deif," Observatory director Rami Abdel Rahman told AFP via telephone on Wednesday.

The Al-Nusra Front has led the offensive against Wadi Deif, according to rebels on the ground.

Insurgents captured the nearby town of Maaret al-Numan, located on the important Damascus-Aleppo highway, in October.

The Observatory, which relies on a network of activists and medics on the ground, says the 21-month conflict has killed 46,000 people, many of them civilians.