Story highlights James Clapper said that ISIS was infiltrating refugees escaping from Iraq and Syria

Clapper warned that ISIS and its eight branches were the No. 1 terrorist threat

Washington (CNN) Top U.S. intelligence officials said Tuesday that ISIS was likely to attempt direct attacks on the U.S. in the coming year and that the group was infiltrating refugees escaping from Iraq and Syria to move across borders.

ISIS "will probably attempt to conduct additional attacks in Europe, and attempt to direct attacks on the U.S. homeland in 2016," Lt. Gen. Vincent Stewart, director of the Defense Intelligence Agency, testified on Capitol Hill Tuesday.

Director of National Intelligence James Clapper, who was also at the Senate Armed Services Committee hearing, estimated that violent extremists were active in about 40 countries and that there currently exist more terrorist safe havens "than at any time in history."

Clapper warned that ISIS and its eight branches were the No. 1 terrorist threat, and that it was using the refugee exodus from violence in Iraq and Syria to hide among innocent civilians in order to reach other countries.

Clapper said ISIS was "taking advantage of the torrent of migrants to insert operatives into that flow," adding that they were "pretty skilled at phony passports so they can travel ostensibly as legitimate travelers."