Two suicide car bombs that exploded in the Syrian capital near a state intelligence compound Thursday killed at least 55 people and wounded another 370, making it the country's deadliest attack since an opposition uprising began 14 months ago.

State media said two booby-trapped cars with 1,000 kilograms of explosives exploded within seconds of each other in the southern Qazaz district in Damascus during the morning rush hour.

The blasts left a scene of carnage with charred vehicles and body parts covering the street. The attack occurred outside the notorious Palestine Branch military intelligence building. The Britain-based Syrian Observatory for Human Rights said most of the casualties were members of the state security forces.

There was no immediate claim of responsibility for the blasts, which further shredded an April 12 cease-fire declared by U.N. envoy Kofi Annan. Opposition leaders said the peace plan was dead, while Western powers insisted it remained the best way forward.

Mr. Annan condemned the “abhorrent” bombings and urged all parties to halt violence and protect civilians. The White House and the United Nations also condemned the attacks.

The Syrian government blamed the attacks on terrorists whom it says are behind the revolt against President Bashar al-Assad.

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

The head of the rebel Free Syrian Army, Colonel Riad al-Asad, denied his group had any role in the bombings.

Syria's main exiled opposition group, the Syrian National Council, accused the government of orchestrating the attack to smear the uprising and scare away a U.N. observer mission.

The chief of the U.N. mission, Robert Mood, a Norwegian general, visited the scene of the attack and condemned the bombings.

“This is a terrible kind of violence that is deplorable. It is the kind of violence that is not deserved by the Syrian people.”

The Syrian foreign ministry sent a letter to the U.N. Security Council urging it to take action against unnamed countries and groups that Damascus accuses of “practicing and encouraging” terrorism in Syria.

The foreign minister of Russia, a key Syrian ally, made a similar accusation about external involvement in Syria's violence.

Speaking on a visit to Beijing, Sergei Lavrov said Thursday some nations, which he did not name, are trying to “explode” the situation. Russia has complained that Western and Arab nations opposed to Mr. Assad have not put enough pressure on Syrian rebels to stop attacks.

U.N. Secretary-General Ban Ki-moon condemned the blasts and repeated his call on all sides in Syria's conflict to fully to comply with Mr. Annan's peace plan that calls for an end to all violence, the protection of civilians, and a dialogue leading to a peaceful resolution of the crisis.

Thursday's attack happened one day after a roadside bombing near the rebellious southern city of Daraa wounded 10 Syrian soldiers who were escorting General Mood and other U.N. monitors. The U.N. personnel were unharmed.

The United Nations says more than 9,000 people have been killed in violence related to the anti-government uprising which erupted in March 2011.