Twin suicide bombers targeted a Damascus police station on Tuesday killing at least 14 people, as the Syrian army launched multiple attacks on rebel positions in the northern province of Aleppo.

The blasts wounded more than 30 people, state media and the Syrian Observatory for Human Rights watchdog said.

They caused widespread damage in the capital's central Marjeh neighbourhood, with state television broadcasting images of blood and broken glass on the pavement near the scene.

"The number of casualties after a twin suicide attack in Marjeh square has risen to 14 dead and 31 injured," said the channel, updating an earlier toll.

"The two suicide explosions took place near a police station," it said.

A shopkeeper nearby struggled to hold back tears as he was interviewed.

"God take revenge against all those who are destroying this country," he said.

The Observatory put the death toll at 15, and said one of the blasts "was caused by a suicide bomb attacker who blew himself up inside the police station."

"A second explosion took place just outside the police station," the Britain-based group said, without elaborating on the nature of that blast.

"The number of casualties is expected to rise because several of the wounded are in critical condition," added the Observatory, which relies on a network of activists, doctors and lawyers for its reports.

Meanwhile, in Aleppo province, the army launched multiple attacks on rebel positions, including insurgent-held areas of the key Minnigh military airbase, the Observatory said.

"Parts of Minnigh military airbase were shelled by regime forces... Rebels are in control of large swathes of the airbase," the Observatory said.

A military source told AFP heavy clashes were raging at the base for a third day, but denied that any part of the airport was under rebel control.

He said the fighting was not part of a broader campaign that the regime has pledged to launch to re-take Aleppo city, large parts of which are in rebel hands, and other parts of the province.

But other areas of Aleppo were under fire, two days after pro-regime media said an army campaign in the province would began within "hours or days."

Regime forces shelled the opposition-controlled villages of Deir Hafer and Al-Bab, and hit the insurgent stronghold of Marea with rockets, the Observatory said.

There were unconfirmed reports of a ground-to-ground missile strike in northern Aleppo province, the Observatory added.

The regime has pledged to focus its attention on Aleppo after winning a strategic victory by retaking the town of Qusayr, in central Homs province.

Backed by fighters from Lebanon's powerful Shiite Hezbollah militia -- a key Syrian regime ally -- the army seized the town near the Lebanese border last week.

The militia's role has raised fears about the growing regionalisation of the conflict in Syria, which began in March 2011.

The fighting has spilled over into Lebanon, where rockets fired from Syria have hit several border regions, and raised concern in neighbouring Israel.

The deteriorating situation in the Golan Heights, a strategic Syrian plateau partly occupied by Israel, has prompted Austria to announce it would withdraw its troops from the UN monitoring force there.

On Tuesday, Austria's defence ministry said the withdrawal of the country's 378 troops stationed in the Golan would begin on Wednesday.

Austria is seen as a cornerstone of the force that monitors the ceasefire line between Israel and Syria.

But the spread of fighting into the ceasefire zone, and the kidnap of several members of the force has prompted the country, along with Japan and Croatia, to withdraw its forces.