Official: Terrorists in Syria Now As Dangerous to US As Those in Yemen Foreign fighters in Syria "number one" topic when talking to European officials.

ASPEN, Colorado -- Syria has risen as a top threat to the U.S. homeland that rivals Yemen's al Qaeda affiliate known for its innovative bombs successfully smuggled aboard airplanes, a top Pentagon official said on Thursday.

"Syria is probably the number one threat -- or, with... threats out of Yemen -- to the American homeland right now and elsewhere in the west," said Michael Vickers, the Pentagon’s Under Secretary for Defense Intelligence.

In answer to a question by moderator Brian Ross of ABC News at a panel at the Aspen Security Forum, an annual gathering of national security veterans, Vickers, himself a former Special Forces operator, said that the foreigners flocking to Syria are hard to track and are a "serious problem."

"Foreign fighters who are Western passport holders, including Americans, a subset of that, numbers in the four digits," Vickers said.

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Since 2010, al Qaeda in the Arabian Peninsula in Yemen has held the top spot in the minds of counter-terrorism officials after the terror group tried unsuccessfully to use four different improvised explosive devices, hidden in underwear on a bomber or inside printer cartridges, in an effort to blow up U.S.-bound commercial passenger and cargo jets.

But a portion of the thousands of foreign fighters joining extremists for training and jihad against Syria's embattled dictator Bashar al Assad over the last few years are viewed as likely to eventually try to attack western targets, many officials have said.

Top U.S. officials have spoken more openly about the threat emanating from Syria this year.

John Carlin, the assistant attorney general for national security at the Justice Department, has established a prosecutors unit to focus on Americans fighting in Syria or aspiring to, because of the enormity of the threat.

"The number of foreign fighters that are already in place in Syria, and a number of other westerners in that group, is one that is unprecedented, and is a larger number than we ever saw in ungoverned spaces in Pakistan and Afghanistan," Carlin said during the panel discussion.

Homeland Security Secretary Jeh Johnson said it's all his European colleagues want to discuss, their fears are so pronounced. "It's number one on the list of discussion topics," he told the security conference.

The tactical threat still remains rooted in the disturbing expertise provided by AQAP's master bomb maker Ibrahim al-Asiri, officials said.

Transportation Security Administrator John Pistole, on a separate panel, said that the type of device he worries about most, "from everything I'm aware of, [is] still a non-metallic IED."

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