WASHINGTON (AP) — A top American intelligence official is acknowledging that the U.S. has difficulty tracking the movements and activities of Westerners in Syria who have joined rebels fighting President Bashar Assad (bah-SHAR' AH'-sahd).

Matt Olsen, who directs the National Counter Terrorism Center, told a House committee Wednesday that intelligence agencies have very little idea where foreign fighters go and what they do once they reach Syria, so they can't estimate how many have joined the Islamic State group or other extremists.

The U.S. is able to track Westerners who travel to and from Syria, Homeland Security Secretary Jeh (jay) Johnson says. But inside Syria, there are intelligence gaps.

The U.S. estimates that about 15,000 foreign fighters have flown to Syria, up from a previous estimate of 12,000.