By Deborah Charles

WASHINGTON (Reuters) - A weakened al-Qaeda has the potential for resurgence in Syria, where the turmoil of civil war could help revive one of the group's close affiliates, a report by a U.S. think tank said on Monday.

"It is too soon to predict the long-term threat posed by al Qaeda and allied groups as the movement is undergoing a transition that may end up proving to be its last gasp; but the right set of circumstances in the unstable Middle East could also revive the network," the Bipartisan Policy Center's Homeland Security Project said in the report called "Jihadist Terrorism: A Threat Assessment."

One of the most effective Syrian rebel groups fighting President Bashar al-Assad is the Nusra Front, effectively a branch of al Qaeda. Opponents of President Barack Obama's plan to attack Syria point out that hitting Syrian government forces in response to a chemical weapons attack last month might end up helping Nusra.

"Al Qaeda's future rises and falls in Syria to some extent," said Peter Bergen, an al Qaeda expert and one of the co-authors of the threat assessment. "We can look around the world - there are actually a lot of places they're not doing well. But clearly they're doing very well in Syria."

The policy center's project is headed by former New Jersey Republican Gov. Thomas Kean and former Indiana Democratic Rep. Lee Hamilton - the co-chairs of the 9/11 Commission Report that analyzed the American preparedness for, and reaction to, the September 11 attacks.

Their report, released ahead of the 12th anniversary of September 11 attacks, said the United States faces a more amorphous terrorist threat, including possible attacks by radicalized individuals.

In Syria, Nusra has begun to operate as a large-scale provider of social services, to garner more support among the people in areas it controls.

"This is something of a first for an al-Qaeda affiliate; developing a Mao-like 'population centric' approach to implementing a successful insurgency," the report said.

Story continues

Arming Syrian rebel groups could allow heavy weapons to fall into the hands of jihadist groups, the report said.

Assad's stockpile of chemical weapons also presents a possible problem if they are accessed by groups like Nusra, the report said. It said the weapons could be used in Syria and also be smuggled out of the country and used in future attacks.

Obama is struggling to convince U.S. lawmakers and foreign governments to back his plan for limited strikes against Assad's forces.

FOREIGN FIGHTERS

Another danger from the Syrian civil war is the presence in Syria of thousands of Muslim radical foreign fighters, including Americans.

"The continued attempts and successes by foreign militant groups to establish support networks in the United States pose a potential future threat, as individuals sending funds to terrorist groups abroad could conceivably be directed to conduct attacks domestically, while American citizens fighting abroad may return to commit terrorism inside the United States," the report said.

Former FBI Director Robert Mueller said last month that the FBI was concerned that Americans may be fighting in Syria and could bring terrorist tactics back to the United States.

But one U.S. national security official recently estimated that only a handful of Americans are fighting in Syria, although other officials have said several dozen U.S. citizens have cycled through the country since the civil war began in 2011.

U.S. and European security officials are more concerned about the traffic in and out of Syria of would-be fighters from European countries, including Britain, France and Germany. Citizens of those countries can easily enter the United States without obtaining an advance visa so fighters who went to Syria from those countries could pose a terrorist threat.

European officials have said that at any one time they believe that as many as 70 to 100 British citizens are fighting in Syria with anti-Assad rebels, many of them with the Nusra Front.

(Editing by Alistair Bell)