Twin car bombs that exploded near a state intelligence compound in the Syrian capital on Thursday killed 55 people and injured nearly 400 more, offering a gruesome new setback to a United Nations- brokered plan meant to temper the violence in Syria.

The explosions, the largest and deadliest of at least five bombs to hit Damascus in more than a year of unrest, risk inflaming the fighting between Syria´s government and opposition forces. Both sides blame each other for the attacks, and neither side has fully complied with the almost four-week old U.N. plan that called for an immediate cease-fire.

Many Syrians say they fear that the bombings, regardless of who perpertated them, offer a frightening preview of the conflict´s next phase. What started 14 months ago as a peaceful uprising evolved partly into an armed conflict. It appears now to echo the violence in neighboring Iraq, where roadside bombs and explosions became a regular feature of the Iraqi insurgency.

"This is a nightmare," a Damascus resident said by telephone, describing a city "paralyzed by fear."

No one claimed responsibility for Thursday´s attacks, which Damascus residents say sent up clouds of black and white smoke visible from across the capital. State media said the two booby-trapped cars, which exploded within seconds of each other, were loaded with more than 1,000 kilograms of explosives. It showed images of charred body parts, crushed cars and the blown-off façade of the intelligence building.

The bombs went off just before 8 a.m. as people walked to work and students went to school, according to government officials, residents and activist groups. Activists identified the building as part of a military compound housing an intelligence services unit called the Palestine Branch, notorious in Syria for its role in the surveillance, detention and what activists describe as the torture of political dissidents.

The Syrian Observatory for Human Rights, a U.K.-based opposition watchdog, said most of the casualties were members of the state security forces. Syria´s government didn´t break down the death toll it provided.

In a statement, the Interior Ministry vowed to "chase the terrorist criminals and those who aid and lodge them," a reference to Arab Gulf states such as Qatar and Saudi Arabia, and neighboring Turkey, which Syrian officials have previously accused of arming and funding the opposition.

Col. Riad al-As´ad, head of the rebel Free Syrian Army, denied his group had any role in the bombings. He said opposition fighters associated with the umbrella group are "complying with the cease-fire totally."

But other opposition fighters have said they will continue to fight the regime as long as government forces are attacking them, underscoring the lack of command in the fragmented armed opposition.

"We hold the regime responsible for these explosions, and the international community must also bear responsibility for the grave escalation in violence," Col. As´ad said in a telephone interview.

Damascus has been hit by a series of suicide bombings since December last year. Two of those—one in March and one in April—have been claimed by a little-known Syrian Jihadist group called Jabhat al- Nusra. According to its online video and text statements, the group appears to have been founded in January. U.S. intelligence officials have voiced concern that Al Qaeda militants may be active in Syria, with some pointing to a stream of insurgents trickling in from Iraq.

The possible emergence of extremists joining the fight against President Bashar al-Assad´s regime, a development that some in the opposition have recognized and condemned, seriously complicates international efforts to pull Syria back from the brink of civil war.

"This potentially toxic mix risks pushing Syria further down the path of civil war—regardless of the intervention of the U.N. and international community," said David Hartwell, a senior Middle East analyst with security intelligence firm IHS Jane´s.

The U.S., France, U.N. and others strongly condemned the attacks.Touring the site of the blasts, Gen. Robert Mood, the head of the U.N. monitoring mission in Syria, called the attacks "terrible violence." Kofi Annan, the special envoy leading a broader peace-plan for Syria that entails the cease-fire, called on all parties to stick to their commitment to stop acts of violence.

"These abhorrent acts are unacceptable and the violence in Syria must stop," a statement from his spokesperson said. "The Syrian people have already suffered too much."

Characterizing the attacks as terrorism, the U.N. Security Council condemned them in the strongest terms. "Any acts of terrorism are criminal and unjustifiable, regardless of their motivation, wherever, whenever and by whomsoever committed," the council said in a statement, which called on Syrian parties to immediately implement all elements of Mr. Annan´s plan.

The U.S. also condemned the attacks, calling on the Syrian regime to "fully and immediately" implement the Annan plan. France said the regime was responsible for "a futile spiral of violence."

No nation has yet publicly suggested pulling the plug on Mr. Annan´s plan, widely described as the last chance to resolve Syria´s crisis. U.S., U.N., and other officials say no other alternatives have been discussed.

—Nada Raad in Beirut contributed to this article. (Copyright © Dow Jones & Company, Inc.) 05/11/12)