The extremists who concern Mr. Brennan are part of a group of Qaeda operatives in Pakistan that has been severely depleted in recent years by a decade of American drone strikes. But the fighters still bring a wide range of skills to the battlefield, such as bomb-building, small-arms tactics, logistics, religious indoctrination and planning, though they are not believed to have experience in launching attacks in the West.
Syria is an appealing base for these operatives because it offers them the relative sanctuary of extremist-held havens — away from drone strikes in Afghanistan and Pakistan — as well as ready access to about 1,200 American and European Muslims who have gone there to fight and could be potential recruits to carry out attacks when they return home. Senior counterterrorism officials have voiced fears in recent months that these Western fighters could be radicalized by the country’s civil war.
New classified intelligence assessments based on information from electronic intercepts, informers and social media posts conclude that Al Qaeda’s senior leadership in Pakistan, including Ayman al-Zawahri, is developing a much more systematic, long-term plan than was previously known to create specific cells in Syria that would identify, recruit and train these Westerners.
Al Qaeda has in the past blessed the creation of local branches in places like Yemen, where an affiliate has tried to strike the United States. But the effort in Syria would signify the first time that senior Qaeda leaders had set up a wing of their own outside Pakistan dedicated to conducting attacks against the West, counterterrorism officials said. It also has the potential to rejuvenate Al Qaeda’s central command, which President Obama has described as being greatly diminished.
The assessment by the United States, however, has some detractors among even its staunchest counterterrorism partners, which also see an increase in Pakistan-based veterans of Al Qaeda among Syrian rebel groups but which disagree over whether they are involved in a coordinated plan to attack the West.
“At this stage, it’s a lot less organized than a directed plan,” said one Western security official. “Some fighters are going to Syria, but they’re going on an ad hoc basis, not at an organized level.”
Most of the operatives identified by intelligence officials are now focused on attacking Syrian government troops and occasionally rival rebel factions. But the fact that these kinds of operatives are showing up in Syria indicates to American officials that Mr. Zawahri is also playing a long game — counting on easy access to Iraq and Qaeda support networks there, as well as on the United States’ reluctance to carry out drone strikes or other military operations against targets in Syria.
“A key question, however, is how using Syria as a launching pad to strike the West fits into Zawahri’s overall strategy, and if he’s soft-pedaling now, hoping to consolidate Al Qaeda’s position for the future,” said one American counterterrorism official. “Clearly, there is going to be push and pull between local operatives and Al Qaeda central on attack planning. How fast the pendulum will swing toward trying something isn’t clear right now.”
The new assessment is not likely to change American policy toward Syria any time soon, but it puts pressure on the Obama administration and its allies because it raises the possibility that Syria could become the next Afghanistan.
Top officials at the F.B.I., the National Counterterrorism Center and the Department of Homeland Security say they are working closely with European allies to track Westerners returning from Syria.
There are perhaps “a few dozen” Qaeda veterans of fighting in Afghanistan and Pakistan in Syria, two top counterterrorism officials said. “What we’ve seen is a coalescence in Syria of Al Qaeda veterans from Afghanistan and Pakistan, as well as extremists from other hot spots such as Libya and Iraq,” Matthew G. Olsen, the director of the National Counterterrorism Center, told a Senate panel in March. “From a terrorism perspective, the most concerning development is that Al Qaeda has declared Syria its most critical front.”
In his first speech as secretary of Homeland Security in February, Jeh C. Johnson put it even more bluntly. “Syria has become a matter of homeland security,” he said.
The Qaeda veterans have multiple missions and motivations, counterterrorism officials say. Like thousands of other foreign fighters, many have been drawn on their own to Syria to fight the government of President Bashar al-Assad.
Many others, like Abu Khalid al-Suri, a Syrian-born veteran of Al Qaeda, were sent by the terrorist group’s central command in Pakistan first to fight Mr. Assad, but also to begin laying the groundwork to use enclaves in Syria to launch attacks against the West, American officials said.
Mr. Suri, who is believed to have been close to Osama bin Laden and to have fought against American forces in Afghanistan and Iraq, was sent to mediate conflicts between Al Qaeda’s main affiliate in Syria, the Nusra Front, and another extremist faction, the Islamic State in Iraq and Syria, which Al Qaeda has disavowed. He was killed in a suicide attack in February by the rival group.
Many of the Qaeda planners and operatives from Afghanistan and Pakistan have clustered in the east and northwest sections of Syria, in territory controlled or heavily influenced by the Nusra Front, intelligence officials said.
Sanafi al-Nasr, a Saudi-born extremist who is on his country’s list of most wanted terrorists, traveled to Syria from the Afghanistan-Pakistan border region late last year and emerged as one of the Nusra Front’s top strategists. Jihadi forums reported that he was killed in fighting last week, but American counterterrorism officials said those reports could not be confirmed.
“Al Qaeda veterans could have a critical impact on recruitment and training,” said Laith Alkhouri, a senior analyst at Flashpoint Global Partners, a security consulting firm that tracks militant websites. “They would be lionized, at least within the ranks, as experienced mujahedeen.”
While these senior Qaeda envoys have been involved in the immediate fight against Syrian forces, American counterterrorism officials said they also had broader, longer-term ambitions.
Without naming Mr. Nasr, James R. Clapper Jr., the director of national intelligence, told a Senate panel in February that a “small nucleus” of Qaeda veterans from Afghanistan and Pakistan in Syria who are “separate from al-Nusra harbor designs on attacks in Europe and the homeland.”
Charles Lister, a visiting fellow at the Brookings Doha Center in Qatar, agreed, saying, “The large majority of Al Qaeda-linked commanders now in Syria are there due to the potential for Syria to be the next jihadist safe haven.”
Hassan Abu Hanieh, a Jordanian expert on Islamist movements, said that launching attacks on Western targets did not appear to be a priority for the Nusra Front now. However, the group’s ideology, or a belief that it was under direct threat, could lead it to attack the West eventually, he said.
“As soon as they get targeted, they will move the battle outside,” Mr. Hanieh said.
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Ben Hubbard contributed reporting from Amman, Jordan.
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