DAMASCUS, Syria – Twin suicide car bombs exploded outside a military intelligence building and killed 55 people Thursday, tossing mangled bodies in the street in the deadliest attack against a regime target since the Syrian uprising began 14 months ago.

The bombings fueled fears of a rising Islamic militant element among the forces seeking to oust President Bashar Assad and dealt a further blow to international efforts to end the bloodshed.

Additional Photos

Syrian firefighters put out fires at one of dozens of cars destroyed by two bombs Thursday in Damascus. A military intelligence building was heavily damaged in the blasts. The Associated Press

The first car bomb went off on a key six-lane highway during the morning rush hour, knocking down a security wall outside the government building and drawing people to the scene, witnesses said. A much larger blast soon followed, shaking the neighborhood, setting dozens of cars ablaze and sending up a gray mushroom cloud visible around the capital.

Syrian state TV video showed dozens of bodies, some charred or dismembered, strewn in the rubble or still inside damaged cars. An Associated Press reporter at the scene saw medics in rubber gloves picking through the site for human remains amid the two craters that were blasted into the asphalt.

The Interior Ministry, which oversees the police and security services, said 55 people were killed and more than 370 were wounded. Officials said suicide bombers detonated explosives weighing more than 1,000 kilograms (2,200 pounds).”The house shook like it was an earthquake,” Maha Hijazi said, standing outside her home nearby.

World powers seeking to halt Syria’s unrest condemned the attack and urged all sides to adhere to a cease-fire brokered by U.N. and Arab League envoy Kofi Annan.

The Obama administration condemned the attack and expressed concern that al-Qaida may be increasingly taking advantage of the country’s prolonged instability.

Defense Secretary Leon Panetta told reporters that U.S. intelligence indicates “an al-Qaida presence in Syria,” but said the extent of its activity was unclear.

“Frankly we need to continue to do everything we can to determine what kind of influence they’re trying to exert there,” Panetta said. He also lamented that a month of efforts to implement a U.N. cease-fire plan haven’t worked.

British Foreign Secretary William Hague said “the onus is on the Syrian authorities to implement a full cease-fire and begin the political dialogue required by the Annan plan,” while the U.N. Security Council said in a statement that “any acts of terrorism are criminal and unjustifiable regardless of their motivation.”

Maj. Gen. Robert Mood, the Norwegian head of a team of observers overseeing the cease-fire, toured the site and said the Syrian people do not deserve this “terrible violence.”

“It is not going to solve any problems,” he said. “It is only going to create more suffering for women and children.”

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