BEIRUT, Lebanon — Twin suicide car bombs that targeted a notorious military intelligence compound shook the Syrian capital, Damascus, on Thursday, killing and wounding hundreds of people and raising the likelihood of extremist elements propelling the conflict to a more treacherous phase.

It was the largest such terrorist attack since the uprising began 14 months ago, with the Health Ministry putting the toll at 55 dead and nearly 400 wounded — civilians and soldiers. The dual explosions forged a hellish landscape of incinerated corpses, burning vehicles and a billowing plume of smoke visible throughout Damascus.

There was no immediate claim of responsibility. But the attack put a spotlight on the growing involvement of Islamic jihadists in the fight against the government of President Bashar al-Assad, particularly those from an Iraqi branch of Al Qaeda that has been openly agitating to join the fray. That prospect raised fears that Syria was heading into the kind of chaos and bloodletting that plagued Iraq and served as a training ground for terrorists.

“There is no question that Al Qaeda in Iraq has attempted to push into the vacuum in Syria,” said Seth G. Jones, a specialist at the RAND Corporation in counterinsurgency, counterterrorism and Al Qaeda in particular. “They are not a majority part of the opposition, and they are not a leading part of the opposition, but they are there.”