Suicide blasts on Monday and a series of other bombings across Syria have renewed concerns that unrest there is giving extremist Islamist groups room to grow, a scenario Western officials fear will make it more difficult to contain the crisis.

The development also complicates efforts to encourage compliance with a United Nations-brokered cease-fire by President Bashar al-Assad—who has long held that the uprising against him is the work of terrorists.

Two suicide bombings ripped through busy districts of the northern city of Idlib early Monday, killing at least nine people and injuring at least 100, the government said. Earlier, rocket-propelled grenades hit the central-bank building in Damascus.

There have been eight suicide attacks reported in Syria since December, including two Monday and a bombing on Friday that killed at least 10 people.

Western and Arab officials and some members of the Syrian opposition say the attacks point to the growing activities of al Qaeda and radical Sunni Islamist groups in operations against the Syrian regime, as the popular uprising against Mr. Assad has developed into armed conflict.

U.S. officials, including Director of National Intelligence James Clapper, have said strikes on security and intelligence buildings in Damascus and Aleppo over the past four months have resembled al Qaeda attacks, and that extremists, mainly al Qaeda insurgents from Iraq, may have infiltrated Syrian opposition groups.

"We are increasingly concerned that these attacks are beyond the control of the Syrian opposition…that other groups are involved," a U.S. official said.

Syrian opposition groups, including the rebel Free Syrian Army, denied any role in Monday´s attacks and blamed the government. Dissidents have consistently said they believe Damascus has staged the attacks to prove it is fighting terrorists, a blanket term the government has used for its opponents.

Yet many activists say extremists are at work—even if in an individual capacity—in antigovernment activities in at least a handful of towns and villages. Many also point to an increasingly visible strain of conservative Islamist ideology making its way even into the protest movement, in slogans and posters.

Conservative Sunni Islamists view Syria´s ruling Alawite sect—an esoteric branch of Islam to which the Assad family and the high ranks of the military and security belong—as apostate.

"What we are seeing is a transformation in the civil war where violence empowers extreme elements of the opposition who have been part of the global Jihadist networks, particularly in Iraq," said Ayham Kamel, a Middle East analyst with political-risk firm Eurasia Group.

U.S. and other Western officials say the presence in Syria of extremists from identifiable groups remains limited, and that the armed opposition is made up largely of army defectors and homegrown civilian fighters.

But some warn that extremists seek a bigger footprint. The U.S. expects to see al Qaeda´s central leadership trying to have a greater hand in operations in Syria and to try to guide its affiliated militants there, said a senior U.S. intelligence official.

"Core al Qaeda was caught somewhat flatfooted in the Arab Spring," the official said. "They´ve had more time on something like Syria" to plan.

Officials also warn that the Syrian regime is likely to use suicide attacks to justify its continued armed crackdown on the opposition, despite its commitment to an April 12 cease-fire brokered by Kofi Annan, the U.N.-Arab League joint special envoy. The opposition, too, has continued to fight government forces since then, with an advance team of 30 U.N. observers documenting attacks from both sides.

The U.N. has authorized 300 observers for the mission, which is due to be in place in Syria over the next few months.

No group took responsibility for Monday´s bombings in Idlib. Some other suicide bombings in Damascus and Aleppo have been claimed by a fledgling Syrian Jihadist group calling itself Jabhat al-Nusra. The group released an eight-minute video last week detailing an attack on a security building in Aleppo in February and naming two of its suicide bombers. It also vowed more attacks against the regime.

Experts said the video, which was posted on jihadist websites, appeared genuine.

Some intelligence analysts said the group appeared to be formed by Syrians who had fought the U.S.-led coalition in Iraq and have since returned to Syria. The group appears to have nevertheless quickly established media communications with the online sites generally used by al Qaeda-linked groups, said Exclusive Analysis, a London-based intelligence firm.

Supporting concerns about a growing radical presence, Lebanese media last week reported the death in Syria of the leader of a Lebanon- based Islamist group, Fatah al Islam, in a bomb-building operation gone awry.

Syrian fighters in the town of Al Qusair, said the Islamist leader, Abdel Ghani Jawhar, died there, confirming news reports. Lebanese officials declined to verify the reports.

In Syria on Monday, state television broadcast images after the attacks in Idlib of crushed cars, casualties amid concrete rubble, and a four-story building with its facade blown off.

A U.K.-based opposition group, the Syrian Observatory for Human Rights, said at least 20 people—mostly members of the security forces— were killed in the bombings, which appeared to target two separate state security buildings. The group also reported a third bombing in Idlib and at least two smaller explosions in Damascus and its suburbs on Monday, marking a day of widespread attacks.

The Free Syrian Army denied a role in the attacks. "These acts are fabricated by the regime to both detract attention from its violent repression and justify its attacks on so-called terrorists," said Maj. Maher al-Nuaimi, a Turkey-based commander with the rebel group. —Siobhan Gorman contributed to this article. (Copyright © Dow Jones & Company, Inc.) 05/01/12)