Infiltrating through borders

The number of foreign fighters traveling to join the Islamic State or rival militant groups in Syria is continuing to grow, a senior U.S. counterterrorism official will tell a congressional hearing on Wednesday.

The United States and its allies believe that more than 20,000 foreign fighters from more than 90 countries have gone to Syria, Nick Rasmussen, director of the National Counterterrorism Center, said in testimony prepared for a hearing of the House of Representatives Committee on Homeland Security.

A senior intelligence official said this compared to an estimate of "more than 19,000" foreign fighters which the U.S. government produced in mid-January.

Foreign fighters still flowing to Syria, U.S. intelligence says