James Clapper
James ClapperReuters

The United States' top intelligence official has warned that the ISIS terrorist group is expanding its operations throughout Europe, and has organized underground networks similar to those behind the recent Paris and Brussels attacks.

Director of National Intelligence James R Clapper Jr. told reporters at a special breakfast on Monday that ISIS networks are operating in Britain, Italy and Germany, among other European states, according to the New York Times. British officials recently said they had foiled seven major terrorist attacks over the past 18 months.

Threats against those three countries in particular have featured repeatedly in ISIS propaganda material, prompting journalists to ask Clapper whether there was any concrete evidence that ISIS had terror networks priming themselves for attacks there.

"Yes, they do. That is a concern, obviously, of ours and our European allies," Clapper stated. "We continue to see evidence of plotting on the part of ISIL (ISIS - ed.) in the countries you named."

Clapper's comments are significant in that they mark perhaps the first such explicit acknowledgement of extensive ISIS terrorist networks operating across Europe by a senior Western intelligence official. Some analysts and terrorism experts had repeatedly warned of the phenomenon previously, particularly in the aftermath of the unprecedented wave of immigration to Europe - much of which was entirely undocumented.

Evidence illustrating the threat came not only via Islamic State's explicit threats, but also from interrogations of terror suspects and "chatter" intercepted by intelligence agencies.

Clapper added that intelligence sharing between the US and European states is crucial in preventing the next attack.

"That, right now, is a major emphasis of ours," he said.