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At least 57 people were killed in Syrian government airstrikes on opposition areas around Damascus Thursday on after rebels fired dozens of mortar rounds at the capital, a monitoring group reported.

The UK-based opposition group, the Syrian Observatory for Human Rights, said at least six children were among those killed in nearly 40 air raids on parts of Eastern Ghouta in Damascus province.

The group, which relies on a wide network of sources within Syria, said at least 140 people were wounded, adding that government forces had also fired surface-to-surface missiles at the area.

The assault came after rebels fired at least 70 rockets and mortar rounds at the capital, killing seven people, including a policeman.

The barrage left usually bustling parts of the city deserted.

"Within minutes, our busy street was empty," said a resident of the Baramkeh neighbourhood, adding that the headteacher of a local school had been forced to take her students to a shelter.

The middle class district is home to several university buildings, as well as the headquarters of state news agency SANA.

In the city centre, traffic was light and many people stayed home from work.

Rebel group Jaysh al-Islam ["Army of Islam"] warned two days ago that it would launch a barrage of missiles at the capital in response to deadly airstrikes on rebel-held Douma in Eastern Ghouta.

Its leader, Zahran Alloush, said Damascus was now considered a "military zone" and a target for the rebels.

The group has carried out such barrages before, including on January 25, when it fired dozens of rockets after a similar warning, killing six.

In Douma, the government airstrikes brought chaos, with an AFP photographer reporting civilians streaming into a makeshift clinic.

Medics, overwhelmed by the number of arrivals, were forced to treat some on the floor.

Among them was an infant, his red and white striped sweater lifted up to allow medics to apply a defibrillator to his bloodied chest.

Eastern Ghouta is a key rebel bastion on the outskirts of Damascus, and has been under siege for nearly two years.

Since mid-2012, the government has conducted frequent air raids on rebel-held areas. It is accused by human rights groups of indiscriminately killing both civilians and fighters.

More than 200,000 people have died in the conflict, which began with anti-government protests in March 2011 and escalated into a civil war that brought jihadists streaming into the country and refugees fleeing out of it.