The latest suicide bombings in the Syrian capital showed an increasing ruthlessness: The attackers struck during rush hour, setting off one blast to draw a crowd before unleashing a much bigger one, killing 55 people and leaving the street strewn with rubble and mangled bodies.

For many, the al-Qaida-style tactics recall those once familiar in the country's eastern neighbor, Iraq, raising fears that Syria's conflict is drifting further away from the Arab Spring calls for political change and closer to a bloody insurgency.

"Syria is slowly but surely turning into another Iraq," said Bilal Y. Saab, a Syria expert at the Monterey Institute of International Studies.

The presence of al-Qaida militants and other extremists adds a wild card element to the Syria conflict that could further hamper international efforts to end it. While world powers and U.N. observers in Syria can pressure the government and the opposition to stick to special envoy Kofi Annan's peace plan, they have no means of influencing shadowy Islamic militants who often don't claim their own attacks.

Western officials say there is little doubt that al-Qaida-affiliated extremists have made inroads in Syria since the popular uprising against President Bashar Assad began 14 months ago. But much remains unclear about their numbers, influence and activities inside Syria.

"We do have intelligence that indicates that there is an al-Qaida presence in Syria, but frankly we don't have very good intelligence as to just exactly what their activities are," U.S. Defense Secretary Leon Panetta told reporters in Washington on Thursday.

Panetta said he didn't know whether al-Qaida was connected to the latest bombings in Damascus.

Amateur videos posted online provide occasional glimpses of extremist activity.

One video posted this week shows a suicide attack that reportedly took place on May 2 in the northern town of Idlib. In the footage, a white van speeds toward an army checkpoint and erupts into a huge ball of flame as it nears the soldiers, sending their bodies flying.

In February, al-Qaida leader Ayman al-Zawahri called on Muslims in neighboring countries to join the uprising, saying Syria's rebels must not rely on the West.

Syria's uprising started in March 2011 with mostly peaceful protests inspired by successful revolts elsewhere calling for political reform. The Syrian government responded with a brutal crackdown, prompting many in the opposition to take up arms to defend themselves and attack government troops.

The U.N. said weeks ago that more than 9,000 people have been killed. Hundreds more have died since.

Thursday's twin blasts in Damascus were the fifth in a string of major attacks in Syrian cities that have clouded the picture of a fight between the opposition and the regime. It was the deadliest yet, in part because it happened on a key thoroughfare during rush hour, while previous bombings were on weekends.

No one has claimed responsibility for the blasts, although a shadowy militant group calling itself the Al-Nusra Front has claimed past attacks through statements posted on militant websites. Little is known about the group, although Western intelligence officials say it could be a front for al-Qaida.

Throughout the conflict, the government and its foes have tried to tar each other with accusations of links to the terror network.

On Friday, Burhan Ghalioun, head of the opposition Syrian National Council, accused the government of cooperating with al-Qaida to carry out the Damascus attacks, using the violence as a way to taint the uprising.

A day earlier, Syria's Ambassador to the United Nations, Bashar Ja'afari, told the U.N. Security Council in New York that al-Qaida, backed by unnamed foreign governments, was behind the attacks.

Late Friday, the leader of Lebanon's Hezbollah militant group and a strong Assad ally warned that attacks like Thursday's could tear Syria apart. Sheik Hassan Nasrallah told supporters via videolink that the same hands that "destroyed and killed in Iraq ... want to destroy Syria today."

Syria's rebels — vastly outnumbered and outgunned by Assad's armed forces and security apparatus — have adopted insurgency tactics, regularly ambushing military checkpoints and convoys.

But Joint Chiefs Chairman Gen. Martin Dempsey differentiated Thursday between extremists and the opposition.

"We do know that there have been extremist elements that are trying to make inroads in Syria," he said. "That is to be distinct from the opposition. I'm not tying those together."

Sometimes, the line between them is unclear.

On Friday, the pro-government TV station Ikhbariya said police shot dead a man driving a truck filled with 1,500 kilograms (3,300 pounds) of explosives. It broadcast video of a police general showing two U.N. observers a minibus holding four large metal containers rigged with explosives.

In the driver's seat, a bearded, bloodied man wore what appeared to be an explosive belt.

"It was similar to the terrorist attack that targeted Damascus yesterday," the general told the observers.

The videos could not be independently verified, and the opposition often accuses pro-Assad media of staging events. Nor was it clear what group — if any — the would-be bomber belonged to.

World powers have backed a peace plan by international envoy Kofi Annan that calls for a truce to allow for dialogue on a political solution to the conflict.

The plan has been troubled from the start, with neither side fully respecting a cease-fire that was supposed to begin on April 12. But the presence of international observers has brought the daily death toll down and halted large government assaults on opposition areas.

U.N. headquarters in New York announced that as of Friday there are 145 military observers and 56 civilian staff deployed in Syria.

Most experts don't expect Annan's plan to fully succeed, and many say large attacks are likely to become more common.

"I think increasingly we'll see less directed bombings and more arbitrary ones that seek to create chaos more than anything else," said Bassam Haddad, director of the Middle East Studies Program at George Mason University.


Hubbard reported from Beirut. Associated Press writers Malcolm Foster in Tokyo, and Bradley Klapper and Lolita C. Baldor in Washington contributed reporting.