Rising Number of Foreign Fighters Joining ISIS

Top American counterterrorism officials are warning that the number of foreign fighters joining ISIS is on the rise.

According to Nicholas Rasmussen, director of the National Counterterrorism Center, more than 20,000 fighters from more than 90 countries have joined the Muslim terrorist group.

"The battlefields in Iraq and Syria clearly provide foreign fighters with combat experience, with training in weapons and explosives, and with access to terror networks that may be ultimately planning attacks which target the West," Rasmussen said at a House Homeland Security Committee hearing Wednesday.

FBI Counterterrorism Assistant Director Michael Steinbach also testified. Steinbach warned that officials don't know how many ISIS fighters have already returned to the United States.

"I would not be truthful if I told you we know about all of the returns. Like Nick said, we know what we know. There is a number that we know we don't know about," Steinbach said.

The terror experts emphasized they know of no specific terror threats to the United States, but that stronger anti-terrorism laws are needed.

Rasmussen said the Islamic State's skillful use of social media is a major reason for its ability to recruit fighters from around the world.