Aspen, Colorado (CNN) -- Concerns about westerners joining the fight in Syria dominated the annual Aspen National Security Forum with top U.S. government officials equating the threat from the stalemated Syrian civil war to that posed by al Qaeda's Yemen affiliate.
Michael Vickers, the Pentagon's undersecretary for intelligence, said there are now believed to be thousands of western passport holders, many with easy visa access to enter the United States, who are among foreign fighters in Syria.
John Carlin, the Justice Department's national security chief, called the threat "unprecedented" and said the department is putting resources to prosecute Americans who try to travel to the region to join terror groups involved in the war.
Vickers called the number of Westerners flocking to Syria to join the jihad "unprecedented" and the number of Syrian fighters with Western passports "is in the four digits."
Director of the National Counterterrorism Center Matt Olsen said about 100 of them are American, "we think."
In addition to the western fighters, Homeland Security Secretary Jeh Johnson said the seizure of territory in Syria and Iraq by the group calling itself the Islamic State posed concern.
The temptation for the group, also called ISIS, would be to try to strike the United States to establish its bonafides as other terror groups have, Johnson said.
"So we're concerned about this type of organization which is very dangerous and probably wants to try and prove itself one way or another, could go further along in its efforts to attack our homeland," Johnson said.
Despite the fact that al Qaeda has renounced ISIS, Johnson suspects there are links between the groups.
"I suspect it's a more complicated picture than that," Johnson said.
TSA weighs in on bombs
John Pistole, administrator of the Transportation Security Administration, said the United States believes al Qaeda bomb maker Ibrahim al-Asiri has trained others who could try to build a device that may be difficult to detect by current screening procedures.
He said his agency works to quickly recalibrate its procedures based on new intelligence on changes in tactics by terrorists.
He cited the example of a new iteration of the underwear bomb that prompted TSA to change screening equipment settings and retrain explosives detection dogs.
That's because the equipment and dogs couldn't detect explosives used in the latter version of the bomb.
One reason the initial underwear bomb used by Umar Farouk Abdulmutallab failed to detonate aboard a U.S.-bound airliner in 2009, Pistole said, was because he wore the explosive underwear contraption for 17 days. Soiling the underwear apparently caused the explosive to fail.
The ISIS threat
Joint Chiefs Chairman Gen. Martin Dempsey said the Islamic Statae of Iraq and Syria is a regional problem that is evolving as a threat to American interests and the U.S. military was working on a strategy to disrupt and ultimately defeat it.
But while the effort to achieve that can be enabled and assisted by the United States, ultimately moderate Sunni Arabs are needed to reject it's extremist ideology, he said.
Dempsey dismissed the of working with Iran against ISIS, saying there is a "lot of American blood on Iran's hands. He also said Syrian President Bashar al-Assad should not be viewed as the "lesser of two evils" and he still needed to leave office.
Russia in Ukraine
On Ukraine, Dempsey said Russia's movement into sovereign country in Europe was an event not seen since Adolf Hitler's invasion of Poland in 1939.
Dempsey said NATO was discussing ways to help boost the Ukrainian military's capability as well as force and basing options to protect neighboring NATO member states.
However, he that feared Russian President Vladimir Putin might have lit a fire he may lose control of in Ukraine, one that may not be contained to Eastern Europe.
Former FBI director Robert Mueller said Friday, "what's happening in Gaza today will feed and fuel the desire for many more to join radical groups."
Mueller was participating in a panel at the Aspen Security Forum discussing the growing flow of foreign fighter, particularly those with western passports, to countries like Syria with the intention of joining the global jihad.
Mueller added that those inspired to join radical groups will not necessarily go to the Gaza strip, but could feel prompted to look join the larger extremist movement elsewhere (for example, in Iraq and Syria).
CNN's Jamie Crawford contributed to this report.