Liberating Mosul from Islamic State (ISIS) could cause a surge of IIS fighters into Europe, it has been warned.

The European Union's security commissioner Julian King said: "The reconquest of the northern Iraqi IS stronghold Mosul can cause violent IS fighters to come back to Europe.

"This is a very serious threat that we have to prepare for," he told German newspaper Die Welt.

King said more than 2,500 people have traveled from European countries to ISIS regions to fight and that some of them could return.

The security official said that while he did not believe there will be an "exodus" of fighters arriving in Europe, "even a small number represents a serious threat".

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