A car bombing near Syria’s ruling party headquarters in Damascus has killed 53 people, according to state media, while mortar rounds exploded near the army’s central command in the city.

It was the third straight day of attacks on the centre of the capital, among the deepest and fiercest on the heart of President Bashar al-Assad’s seat of power during the civil war.

The car bombing was the deadliest attack inside Damascus in nine months and within hours, two other bombings and a mortar attack on the military compound followed.

While no one group has claimed responsibility, the attacks suggest that rebel fighters who have got bogged down in their attempts to storm the capital are resorting to guerrilla tactics to loosen Assad’s grip on the capital.

In the southern town of Daraa, where Syria’s uprising began nearly two years ago, British-based activists said 18 people were killed in an air strike, including eight rebel fighters, three medics, one woman and one young girl.

The day’s deadliest attack struck a main street on the edge of central Mazraa neighbourhood, near the headquarters of Assad’s Baath party and the Russian Embassy, as well as a mosque, a hospital and a school.

TV footage of the blast site showed firemen dousing a flaming car with hoses and lifeless and dismembered bodies blown on to the grass of a nearby park.

The state news service, Sana, published photos showing a large crater in the middle of the rubble-strewn street and charred cars holding blackened bodies.

Witnesses said a car exploded at a security checkpoint between the Russian Embassy and the central headquarters of Assad’s ruling party.

“It was huge. Everything in the shop turned upside down,” one local resident said. He said three of his employees were injured by flying glass, which killed a young girl who was walking by when the blast hit.

“I pulled her inside the shop but she was almost gone. We couldn’t save her. She was hit in the stomach and head,” he said.

Ambulances rushed to the scene of the blast, which shattered windows and sent up a huge cloud of smoke visible throughout much of the city, witnesses said. State TV called it a “terrorist” attack by a suicide bomber. It said at least 53 people were killed and more than 200 wounded.

Britain-based activist group Syrian Observatory for Human Rights said at least 42 people were killed, most of them civilians. Some members of the Syrian security forces were also killed, it said.

There was no way to immediately reconcile the differing death tolls. The bombing appeared to be the second most deadly in the Syrian capital since the uprising against Assad began 23 months ago.

Fifty-five people were killed in the first, a double suicide bombing outside an intelligence building in May 2012.

The most extreme of Syria’s rebel groups, Jabhat al-Nusra, claimed responsibility for that and other bombings that have struck targets associated with the regime but also killed civilians.

There was no immediate claim of responsibility for yesterday’s attack.

Russia’s state-owned RIA Novosti news agency quoted a Russian Embassy official as saying the Embassy building had been damaged in the blast but no one was hurt.

Among those wounded by flying glass was Nayef Hawatmeh, the leader of the Democratic Front for the Liberation of Palestine, a radical Damascus-based Palestinian group.

He was taken to hospital and later released.

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