Director of National Intelligence James Clapper (right) testifies at a House Permanent Select Committee on Intelligence hearing on "World Wide Cyber Threats" in Washington, Sept. 10, 2015. REUTERS/Gary Cameron

Attacks by "homegrown" terrorists are among the most imminent security threats facing the United States in 2016, along with dangers posed overseas by Islamic State group and cybersecurity concerns, the top U.S. intelligence official said Tuesday.

In his annual assessment of threats to the United States, Director of National Intelligence James Clapper warned that fast-moving cyber and technological advances "could lead to widespread vulnerabilities in civilian infrastructures and U.S. government systems."

Terrorism remains a top concern in President Barack Obama's last year in office, Clapper said in prepared testimony before the Senate Armed Services and Intelligence Committees.

Clapper outlined an array of other threats, from Russia and North Korean nuclear ambitions to instability caused by the Syrian migrant crisis.

"In my 50-plus years in the intelligence business I cannot recall a more diverse array of crises and challenges than we face today," Clapper said during the hearing.

The Islamic State terrorist organization, aka ISIS, poses the most danger among militant groups because of the territory it controls in Iraq and Syria, Clapper said. It also has demonstrated "unprecedented online proficiencies," he said.

While the United States "will almost certainly remain at least a rhetorically important enemy" for many foreign militant groups, "homegrown violent extremists ... will probably continue to pose the most significant Sunni terrorist threat to the U.S. homeland in 2016," Clapper said, referring to Sunni Muslim jihadists.

"The perceived success" of attacks by such extremists in Europe and San Bernardino, California, "might motivate others to replicate opportunistic attacks with little or no warning, diminishing our ability to detect terrorist operational planning and readiness," Clapper said.

A married couple inspired by Islamic militants shot and killed 14 people in San Bernardino in December.

General Vincent Stewart, director of the Defense Intelligence Agency, told the Senate Armed Services Committee that ISIS aims to conduct more attacks in Europe during 2016 and has ambitions to attack inside the United States.

The group is taking advantage of the refugee flow from Syria's civil war to hide militants among them and is adept at obtaining false documentation, Clapper said.

Al Qaeda affiliates, most notably the one in Yemen known as Al Qaeda in the Arabian Peninsula, have proved resilient and are positioned to make gains this year despite pressure from Western counterterrorism operations, Clapper said.

He cited threats from Russia's increasingly assertive international policies, saying: "We could be into another Cold War-like spiral."

U.S. intelligence assesses that North Korea is committed to developing a long-range nuclear armed missile that can reach the United States and has carried out some steps toward fielding a mobile intercontinental ballistic missile system, Clapper said.