Iraqi troops backed by U.S. soldiers and Kurdish Peshmerga launched the operation Monday | Ahmad Al-Rubaye/AFP via Getty Images EU anti-terror chief: Mosul offensive could send ISIL fighters to Europe Europe must better secure its borders to deal with threat, commissioner said.

Julian King, the EU commissioner for the security union, said the bloc must secure its borders against European Islamic State fighters returning home as a result of the new military offensive against the group in the Iraqi city of Mosul.

"Around 2,500 ISIL fighters from EU countries remain on the battlefield," King warned in an interview with German daily Die Welt. "The recapture of the northern Iraqi ISIL-stronghold Mosul can prompt violent ISIL fighters to return to Europe."

While King said he did not expect a "mass exodus" of ISIL fighters, "even a small number poses a serious threat and we have to be prepared for that."

The EU must better protect its borders and "increase our resilience to the threat of terrorism" in response, the commissioner said.

King's comments echo those made by U.S. officials that ISIL-trained fighters are likely to flee Mosul as it is recaptured and return home to Europe and to a lesser extent the U.S., where they may carry out terror attacks.

Iraqi troops backed by U.S. soldiers and Kurdish Peshmerga launched the operation to retake Mosul, from which ISIL’s leader Abu Bakr al-Baghdadi declared a caliphate in 2014, in the early hours of Monday.