The commanding general of the U.S. Army in Europe expressed concerns on Thursday over the growing number of Europeans being recruiting by Islamic State (ISIS) militants amid fears they could return to launch attacks within the region, The Hill reports.

The Islamic State forces outreach has seen thousands of European fighters head to Syria, and keeping track of suspects has become impossible, Maj. Gen. Roger Piatt told The Hill.

"We see recruiting kind of accelerating," Piatt told media on Thursday, The Hill reported. "It's a concern. ... We've seen it all the way throughout Europe and [it's] impossible to track every single person where they are."

Concerns continue over securing Turkey's border, even as the nation has not sought outside support for that effort, The Hill noted of ISIS access into Syria. Normal security operations have been increased at certain places within Europe as fears of terrorism rise, The Hill noted.

Six attempted ISIS attacks have reportedly occurred in Europe, Breitbart reported, citing accounts from the U.K.'s Express.

The news was revealed by British Prime Minister David Cameron in a speech to Parliament.

"The point I would make even today to the British people is: Be in no doubt about the threat that so-called Islamic State poses to us." Cameron said. "We have already seen something like six planned attacks in the countries of the European Union from [ISIS], including of course that appalling attack in the Brussels Jewish museum, where innocent people were killed. That flows directly from this organization."

Protests have erupted in some cities in Europe as the Kurds express outrage over the Islamic State spread, the New York Post reported. Nine died as violent protests occurred in Turkey, the Post noted.

The violent clashes have occurred as Kurds have sought support over attacks by Islamic State forces in Kobani, the U.K.'s Daily Mail noted.

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