The United States faces terror threats from across the globe, according to the country’s top intelligence official — but few are more pressing than “homegrown” extremists in the U.S.

Americans who launch attacks against their own neighbors “will probably continue to pose the most significant Sunni terrorist threat to the U.S. homeland in 2016,” Director of National Intelligence James Clapper James Robert ClapperOn China, Biden is no Nixon — and no Trump The Hill's 12:30 Report - Speculation over Biden's running mate announcement Trump slams former intelligence officials to explain 'reluctance to embrace' agencies MORE said in testimony prepared ahead of a Senate hearing on Tuesday.

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“The perceived success of attacks by [homegrown violent extremists] in Europe and North America, such as those in Chattanooga and San Bernardino, might motivate others to replicate opportunistic attacks with little or no warning, diminishing our ability to detect terrorist operational planning and readiness,” Clapper added.

In particular, Clapper warned, Americans will be inspired by the “highly sophisticated media” from the Islamic State in Iraq and Syria (ISIS), even if the extremist group does not directly launch attacks in the U.S.

Last year’s violence in Tennessee and California killed a total of 19 people, plus the shooters. The incidents appear to have been motivated by messages from overseas Islamic extremists, but were not connected to a larger cell.

Yet ISIS continues to attract thousands of foreign fighters to its cause, Clapper warned. Since the Syrian civil war began in 2012, more than 36,500 foreigners have traveled to join its self-declared caliphate in the Middle East. Of those, at least 6,600 are from Western countries, the spy chief warned.

“ISIL's branches continue to build a strong global network that aims to advance the group's goals and often works to exacerbate existing sectarian tensions in their localities,” he claimed, using an alternate acronym for the organization.

In his remarks, Clapper also warned that al Qaeda, while “significantly degraded,” still “aspires to attack the U.S. and its allies.”

Iran also poses a “continuing terrorist threat to U.S. interests and partners worldwide,” Clapper noted, even after implementation of the nuclear deal this year.

Also on Monday, the House Homeland Security Committee released its monthly “snapshot” of threats facing the U.S., which painted a similar picture.

“ISIS continues its global expansion on the shores of the Mediterranean Sea and the still-dangerous al Qaeda in the Arabian Peninsula seizes greater territory in Yemen,” committee Chairman Michael McCaul (R-Texas) said in a statement. “This year is on track to be as dangerous as – if not worse than – 2015 for the American homeland and our national security.”