Islamic extremists have more secret cells hidden throughout Europe that could be deployed in terror attacks against civilians, the top U.S. intelligence official warned on Monday.

Director of National Intelligence James Clapper said during a breakfast sponsored by the Christian Science Monitor that terror groups have additional clandestine cells similar to those that mobilized during attacks in Paris and Brussels in recent months.

In particular, Clapper certified that U.S. intelligence officials “continue to see evidence on the part of ISIL” in Germany, England and Italy. ISIL is an alternate acronym for the Islamic State in Iraq and Syria (ISIS).

“That is a concern, obviously, of ours and European allies. I assure you we are doing all we can to share with them,” he added.

Following the deadly attacks in Paris and Brussels, focus on sharing information has ratcheted up considerably to ensure that intelligence and law enforcement agencies across Europe are on the same page. Some critics said after the March Brussels attack that Belgian officials were not adequately prepared to deal with the rising threat of radicals tied to ISIS.

Sharing information “is a major emphasis of ours,” Clapper said on Monday.

European security officials have cracked down on suspected extremists at a dramatic rate since the violence in Paris and then Brussels, arresting multiple people believed to have links to ISIS.

The extremist group has increasingly promised that the attacks in Europe are part of a larger campaign to inflict violence far from its self-proclaimed caliphate in the Middle East, worrying officials and the public in Europe and the U.S.