A car bombing near Syria's ruling party headquarters in Damascus killed 53 people on Thursday, 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 center of the capital, among the deepest and fiercest on the heart of Bashar Assad's seat of power during the civil war.

The car bombing, which injured at least 200 people, 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 gotten bogged down in their attempts to storm the capital are resorting to guerrilla tactics to loosen Assad's grip on the capital.

The fighting in Damascus also follows a string of tactical victories in recent weeks for the rebels — capturing the nation's largest hydroelectric dam and overtaking airbases in the northeast — that have contributed to the sense that the opposition may be gaining some momentum.

The day's deadliest attack struck a main street on the edge of central Mazraa neighborhood, 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 into 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.

'Turned upside down'

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

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The car bomb shook central Damascus near the headquarters of the ruling Baath party and the Russian Embassy. (SANA/Associated Press)

"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 that 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, speaking on condition of anonymity for fear of retribution for speaking with foreign media.

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.

The Britain-based activist group the Syrian Observatory for Human Rights said at least 59 people were killed, most of them civilians. Some members of the Syrian security forces were also killed, it said.

UN Envoy Renewed

The United Nations and the Arab League have extended the mission of their joint envoy to Syria through the end of the year.

UN spokesman Martin Nesirky announced Thursday that Lakhdar Brahimi's contract has been renewed through the rest of 2013. It had been set to expire Friday.

International efforts to end Syria's war have so far failed. The UN says at least 60,000 people have been killed and millions have fled.

Brahimi said this week that an opposition offer to negotiate "challenges the Syrian government to fulfil its often-repeated assertion that it is ready for dialogue and a peaceful settlement."

-Associated Press

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 of 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 Thursday'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. An official at his office said Hawatmeh was wounded in the hands and face from flying glass. He was taken to hospital and later released.

Army command centre hit

In a separate attack, Syrian state TV said mortar shells exploded near the Syrian Army General Command in central Damascus, causing no casualties. The station said the building was empty because it was under renovation.

The Britain-based activist group Syrian Observatory for Human Rights said two mortar rounds struck near the building but did not report casualties.

On Wednesday, two mortar shells exploded next to a soccer stadium in Damascus, killing one player. The day before, two mortar shells blew up near one of Assad's three palaces in the city, causing only material damage.

Between the car bomb and the mortar attack near the army command, a security official reported another blast in the capital's northeastern Barzeh neighborhood.

The Observatory, which relies on a network of anti-regime activists inside Syria, said two car bombs had exploded near security centers in Barzeh, followed by intense clashes between rebels and security forces..

Damascus has so far mostly avoided the large-scale violence that has destroyed other Syrian cities, though deadly car bombings have targeted government buildings in the capital.

In May 2012, twin car bombs exploded outside a military intelligence building, killing 55 people in the deadliest attack against a regime target in the capital since the uprising began 23 months ago.

And in July, rebels detonated explosives inside a high-level crisis meeting in Damascus that killed four top regime officials, including Assad's brother-in-law and the defense minister.

Airstrike

In the southern town of Daraa, where Syria's uprising began nearly two years ago, the Observatory said the 18 people killed in the airstrike included eight rebel fighters, three medics, one woman and one young girl.

A video posted online said to be of the event showed the bodies of dead and wounded people being loaded in to the back of trucks and moved to another location. Some were bloody and had bandaged heads, while others were carried out on stretchers.

Syria's conflict began in March 2011 with political protests against the government and has since evolved into a civil war between Assad's regime and hundreds of rebel groups seeking to topple it. The UN says some 70,000 people have been killed in the conflict so far.

International diplomacy has failed to slow the fighting.