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.