Story highlights As many as 1,900 foreign fighters who went to Iraq and Syria have gone back to Europe

Forming new government in Libya is key to countering ISIS, diplomats say

Washington (CNN) The terror threat in Europe is at as high a level as it's ever been, a U.S. official told CNN Wednesday, and nearly 2,000 foreign fighters who went to Iraq and Syria from Europe have returned home -- with some potentially plotting new attacks.

The latest European and U.S. estimates show as many as 1,900 of those foreign fighters are back in Europe after stints in Iraq and Syria, the official said. The estimate covers the last few years since intelligence services across the world began tracking these movements.

Most of the fighters returning -- but not all of them -- joined ISIS while in the terror group's self-declared caliphate and remain committed to the group. Others became disenchanted and fled back to Europe. It's not clear how many of those who returned are plotting new attacks, the official said.

JUST WATCHED French official: ISIS running fake passport 'industry' Replay More Videos ... MUST WATCH French official: ISIS running fake passport 'industry' 03:30

"The threat stream is as high as it's ever been" in Europe, the official said, and the tracking of possible plots and threats across that continent has not diminished since the attacks last year in Paris and the holiday season.

There is also a "major risk" of ISIS operatives mixing in with refugees and trying to travel to Italy from North Africa and other parts of the Mediterranean, a second U.S. official told CNN. The Italians, this official said, are expressing their alarm about this issue to the United States and other European allies.