A top U.S. intelligence official warned Tuesday that the Islamic State (ISIS) may plan to attack the U.S. this year and that the terrorist group may infiltrate refugees from Iraq and Syria to go to other countries.

Lt. Gen. Vincent Stewart, director of the Defense Intelligence Agency (DIA), testified on Capitol Hill, saying ISIS "will probably conduct additional attacks in Europe and attempt to direct attacks on the U.S. Homeland in 2016," according to CNN.

"[ISIS] will likely increase the pace and lethality of its transnational attacks as infrastructure and capabilities mature. It will purposefully attempt to stoke sectarian conflict between Shia and Sunni, and between the West and Islam, to create the chaotic environment in which it thrives," Stewart said during the hearing.

He said the DIA expects ISIS leaders in Syria to be increasingly involved in directing attacks rather than just encouraging lone attackers.

"[ISIS'] foreign fighter cadre is core to its external attack capability, and the large number of Western jihadists in Iraq and Syria will pose a challenge for Western security services," he said.

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

He said ISIS and its eight branches are the No. 1 terrorist threat and the terrorists are using refugees from Syria and Iraq to infiltrate other countries.

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

ISIS has reportedly seized Syrian passport facilities to make passports.

The "Worldwide Threat Assessment of the U.S. Intelligence Community" said "approximately five dozen" ISIS-linked people were arrested in the U.S. in 2015.

Clapper said more than 38,200 foreign fighters, including at least 6,900 from Western countries, have gone to Syria from more than 100 countries since 2012.

He also said "al Qaeda affiliates are positioned to make gains in 2016."

On North Korea, Clapper expressed concern about the January nuclear test and Saturday's satellite launch, saying that the Communist country was "committed to developing a long-range nuclear-armed missile that's capable of posing a direct threat to the United States."