Gilles de Kerchove told The Associated Press in an interview that there hasn't been a massive flow of IS fighters returning to Europe as many had feared following the group's loss of territory in Syria and Iraq, adding that the intelligence services describe the fighters' return as "more a trickle than a flow."
However, people inspired by the ideology remain a threat and the group is likely to entrench in other parts of world with "weak governance" such as Libya or Afghanistan, the EU official said.
IS has been driven from more than 96 per cent of the large parts of Iraq and Syria it once held, crushing its goal of establishing a "caliphate" in the region.
Raqqa, the group's de-facto capital in Syria, fell to Kurdish-led forces on October 17, four months after operations to reclaim it began. The city was the group's hub of operations, and its capture was a major symbolic blow.
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"That does not mean that the game is over. We still have to address the ideology. More and more, we see in Europe (people) inspired by terrorism, home-grown terrorism."
He said Islamic State "will probably develop in one way or the other in some parts where you have weak governance. Either more weak governance like in Libya or Afghanistan or where it's more difficult to police, like in the Sinai."
An attack on a mosque in Egypt's northern Sinai region last week killed more than 300 people.
He praised Turkey's efforts to fortify its border with Syria, through the construction of a wall and improved policing.
"I wouldn't say it's impossible to cross the border but it's now 10 times more difficult than it was before," he said.
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