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.

With regime forces gaining ground, France said the nearly 27-month conflict, which is estimated to have killed at least 94,000 people, is at a "turning point" and that it was time to review whether to arm the opposition.

Forces loyal to President Bashar al-Assad, including thousands of Lebanese Hezbollah fighters, have overrun rebel fighters in central Syria in the past week, including the strategic town of Qusayr.

"There are lessons to be drawn from what happened in Qusayr and what is happening in Aleppo," said French foreign ministry spokesman Philippe Lalliot.

"We are at a turning point in the Syrian war. What should we do under these conditions to reinforce the opposition armed forces? We have had these discussions with our partners, with the Americans, the Saudis, the Turks, many others.

"We cannot leave the opposition in the current state."

The issue of military support for the opposition is likely to top the agenda when US Secretary of State John Kerry meets in Washington with British counterpart William Hague on Wednesday.

Assad's regime, dominated by his Alawite sect of Shiite Islam, appears to have gained the upper hand against mainly Sunni Muslim rebels, buoyed by military support from its Shiite allies, Hezbollah and Iran.

Damascus has also benefited from political support on the international stage from Russia, which supplies it with weapons and has blocked UN Security Council resolutions condemning it.

But Russian President Vladimir Putin said on Tuesday that he always believed that Assad should have implemented political reforms that could have averted the current bloodbath.

Tuesday's blasts in Damascus shed more blood, killing at least 14 people, wounding 31, and causing widespread damage in the Marjeh neighbourhood, state media and the Syrian Observatory for Human Rights said.

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

Syria's cabinet denounced the attack, saying "armed terrorist groups and those behind them have failed completely because of the victories achieved by our brave army".

In Aleppo province, the army launched multiple attacks on rebel positions, including areas of the Minnigh airbase held by insurgents, the Observatory said.

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

A military source told AFP heavy clashes were raging at the base for a third day, but denied 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 retake Aleppo city, large parts of which are in rebel hands.

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

The regime has pledged to focus its attention on Aleppo since its triumph in Qusayr, a town on routes to Lebanon and the Alawite coastal heartland that the rebels had held for a year.

Hezbollah's role in that devastating 17-day assault has raised fears about the growing regionalisation of the conflict in Syria, which began in March 2011.

The deteriorating situation on the Golan Heights has prompted Austria to say it will withdraw its troops from the UN monitoring force on the strategic Syrian plateau, most of which is occupied by Israel.

Austria's defence ministry said the withdrawal of its 378 troops would begin on Wednesday.

The international community, led by Russia and the United States, has been pushing for a peace conference in Geneva to discuss a political solution.

But preparations have stalled over which countries will participate, with Iran saying it has been invited to the conference, despite Western opposition.

"Ten days ago, we received a verbal invitation to take part in this conference," Deputy Foreign Minister Hossein Amir Abdollahian told reporters in Moscow, without saying who extended the invitation.