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U.S. officials warn of increase of ISIS foreign fighters

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Washington (CNN)The number of foreign fighters traveling to join ISIS’s ranks is increasing at an alarming rate, a top U.S. counterterrorism official is warning.

Nicholas Rasmussen, director of the National Counterterrorism Center, said more than 20,000 fighters, from more than 90 countries, have traveled to the ISIS battlefield, according to testimony CNN obtained in advance of a House Homeland Security Committee hearing scheduled for Wednesday.

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“The rate of foreign fighter travel to Syria is unprecedented,” Rasmussen said in the testimony. “It exceeds the rate of travelers who went to Afghanistan and Pakistan, Iraq, Yemen or Somalia at any point in the last 20 years.”

Of those fighters, an estimated 3,400 are assessed to come from western countries, including more than 150 from the United States, officials say.

Rasmussen is set to appear alongside Francis X. Taylor, under secretary for the Office of Intelligence and Analysis at the Department of Homeland Security, and FBI counterterrorism assistant director Michael Steinbach, at the hearing on efforts to combat violent extremism.

ISIS’s prolific exploitation of social media plays a prominent role in its ability to recruit fighters from around the world, Rasmussen said. The group “has proven far more adept than core al Qaeda — or any of al Qaeda’s affiliates — at using new media tools to reach a broader audience,” he said.

While the three officials maintain there is no specific or credible imminent threat the U.S. homeland, Taylor highlighted recent terrorist attacks in Paris, Sydney and Ottawa as a testament that attacks are moving beyond the borders of Iraq and Syria, and lessons gained on the battlefield could be put to use elsewhere.

“Their experience and successes on the battlefields in Syria and Iraq have armed this group with advanced capabilities that most terrorist groups do not have,” Taylor said in his prepared statement.

With the United States and its allies looking for ways to staunch the flow of fighters to and from the battlefield, officials say stronger counterterrorism laws are needed.

As fighters have exploited different modes of transportation from land, sea and air, Rasmussen cites Turkey’s visa-free travel arrangements with more than 69 governments that allow for the passage of fighters to move through Turkey’s domestic airports, while fighters from the Caucauses are able to move across multiple borders in hopes of crossing the Turkish-Syrian border.

While Turkey has recently added efforts to deny entry to potential foreign fighters, Rasmussen says “significant work remains” in the effort to prevent foreign fighters to cross borders either to or from the battlefield.

Rep. Michael McCaul, R-Texas, the chairman of the House Homeland Security Committee, also expressed alarm in his anticipated opening statement, about small-scale radicalization in the United States that may be hard to detect. The most glaring example is the 2013 Boston Marathon bombing, but there have been other so-called ‘lone-wolf’ attacks that were successfully thwarted in recent times.

“I wrote to the President recently as part of my ongoing investigation and raised concerns that we have no lead agency in charge of countering domestic radicalization and no line item for it in the budgets of key departments and agencies,” McCaul said in his prepared opening statement. “I am also concerned that the few programs we do have in place are far too small to confront a challenge that has grown so quickly.”