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Syria also stands alone among the places where foreign fighters have joined the jihadist movement over the past decade, including Somalia, Yemen, Libya and Mali, he added.

“There’s only one place it has stuck, and that is Syria,” he said.

“Each of these conflicts in the region generates a new series of networks — funders, suppliers, moneymen — and all of these networks get turned to whatever new conflict that comes up,” Mr. Watts said. “Someday the Syria conflict is going to stop raging, but these networks are going to be there, and the fighters are still going to be there.”

William McCants, a Middle East researcher at the Brookings Institution, said Syria’s strategic location and porous borders make it a particularly attractive safe haven for jihadists.

Syria is also right next door to Israel, and a cherished dream for jihadis of all stripes is to finally take the battle to Israel,” Mr. McCants said.

Analysts said al Qaeda is operating unfettered in Syria, in part because of the Obama administration’s limited counterterrorism tools: It cannot use drones without violating Syrian airspace, and it cannot deploy special operations forces in the country without joining and escalating the war.

“The reason we’re not intervening in Syria is because we realized after Libya [that] we can’t keep control of everything the way we would like, and it opens up a can of worms,” said Mr. Watts. “We have let this be a safe haven, and that is why they are going there.”

Meanwhile, the U.S. has appealed to regional partners to help stem the flow of foreign fighters into Syria, and now plans to expand its counterterrorism support to Iraq, which is besieged by al Qaeda attacks, officials say.

Al Qaeda fighters are bringing heavy weaponry from Syria into Iraq and have become an increasing threat in the past eight months, a senior administration official told reporters Wednesday during a background briefing on Iraqi Prime Minister Nouri al-Maliki’s visit to Washington this week.

“It is a fact now that al Qaeda has a presence in western Iraq, and it has a presence in terms of camps and training facilities and staging areas that the Iraqi forces are unable to target effectively,” the official said.

Al Qaeda forces in Iraq and Syria are so linked that U.S. officials now consider that al Qaeda in Iraq has essentially become one entity with the self-styled ISIS (the Islamic State of Iraq and the Levant). Another al Qaeda affiliate is fighting in Syria — Jabhat al-Nusra.