WASHINGTON — The Obama administration on Thursday condemned a suicide attack in Syria's capital that killed 55 people 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.

At the White House, press secretary Jay Carney condemned Thursday's twin suicide blasts in Damascus, saying "attacks like these that result in the indiscriminate killing and injury of civilians are reprehensible and cannot be justified."

"They also remind us of the urgent necessity of achieving a political solution before it is too late," he said.

Syria's government said the attack wounded more than 370 people and described it as the deadliest bombing so far in the country's 14-month uprising. It occurred near a military intelligence building, the apparent target.

President Barack Obama has long called for Syrian leader Bashar Assad to leave power, blaming his government for its brutal military repression of political opponents and ongoing fighting with rebels, many of whom have taken up arms to defend themselves. The U.N. says more than 9,000 people have died.

But the increasingly violent response to the Assad regime's crackdowns is creating headaches for the Obama administration. While it has repeatedly demanded that the Syrian government lay down its arms, it has also increasingly been forced to condemned violence from elements within the anti-Assad camp.

"We do not believe this this kind of attack that you saw in Damascus is representative of the opposition," Carney said. "There are clearly extremist elements in Syria, as we have said all along, who are trying to take advantage of the chaos in that country — chaos brought about by Assad's brutal assault on his own people."

Panetta said he had no information about who was responsible for the latest attack.

Joint Chiefs Chairman Gen. Martin Dempsey echoed the assertion, but said "we do know that there have been extremist elements that are trying to make inroads in Syria."

The government has blamed terrorists for the bombings, but also for peaceful protests. Opposition leaders and activists routinely blame the Assad regime for orchestrating attacks to demonize the opposition. There was no immediate claim of responsibility for Thursday's bombings.

State Department spokeswoman Victoria Nuland said the onus was with the Assad regime to "lead the way in demonstrating its commitment to the cease-fire."

"We think that that would set the tone and we would not be seeing these kinds of violent episodes elsewhere in the country," she said. "It is the Assad regime that created this climate of violence that is causing not only folks to take up arms in defense, but is also providing an environment, potentially, for mischief to be made by others who don't favor peace in Syria."

The State Department also announced an additional $6.5 million in humanitarian aid for Syria. That brings total U.S. assistance to $40 million since the crisis began in March 2011.