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 extremist element among the forces seeking to oust President Bashar al-Assad and dealt a further blow to international efforts to end the bloodshed.

The first car bomb went off on a 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.

The Interior Ministry, which oversees the police and security services, said 55 people were killed and more than 370 hurt. Officials said suicide bombers detonated explosives weighing more than 2,200 pounds.

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.

"In order to prevent another escalation of violence, we continue to call on the Syrian regime to fully and immediately implement the Annan plan," State Department spokeswoman Victoria Nuland said in Washington.

The blast was the largest and deadliest yet in a series of bombings targeting state security buildings since December.

The government blamed the attack on terrorists it says are driving the uprising, which has grown into the strongest threat to the Assad dynasty in its four decades in power.

The Syrian Foreign Ministry sent letters to U.N. Secretary-General Ban Ki-moon and the head of the Security Council, asking the body "to take steps against states, parties and media outlets that practice and encourage terrorism," Syria's state news service said.

Syria's U.N. ambassador, Bashar Ja'afari, told the Security Council that a second bombing Thursday, in Aleppo, also killed civilians and damaged property.

The Britain-based Syrian Observatory for Human Rights said five intelligence officers were killed when a bomb targeted their car in Aleppo. It was unclear if this was the same incident.

There was no immediate claim of responsibility for the Damascus attack.

A leader of the Free Syrian Army, an umbrella group of anti-regime militias, condemned the attack and denied the group was involved.