EU nationals fighting for IS drop by half

Up to 20 percent have died and up to 35 percent have already returned to Europe, according to figures collected by EU's counter-terrorism coordinator.

"Those having already returned could pose a threat to security, though their motivation to travel to Syria in the first wave may have been more to protect Muslims than to join Daesh," notes the paper.

An Iraq-led military campaign launched in October, with air and ground support from the United States, has seen the territory controlled by so-called Islamic State reduce dramatically. Fears are mounting that some EU nationals may flee the assault and instead launch attacks in Europe.

But the paper says any immediate massive return is regarded "as unlikely" and that the extremist fighters are instead more likely to remain and form resistance pockets.

"FTFs [foreign terrorist fighters] will also continue to die in Syria and Iraq, especially in the context of the battles in Raqqa and Mosul," notes the paper.

The paper also notes that some foreign fighters and their families still in Iraq or Syria want to return but are unable to do so because they are viewed as defectors by the Islamic State.

EU states are mulling ideas on what to do with those, like women and children, who may been dragged along to join the extremists.

Hundreds of children are also thought to have been born or raised in the Caliphate and are likely to either have been victims of violence or been traumatized after witnessing violence. Some women are also likely to be victims of sexual abuse.

At the same time, fewer and fewer people are travelling to take up arms in Syria and Iraq given the military operations and, possibly, an loss of appeal towards IS.

A source in contact with intelligence services also recently told EUobserver that with the evolving situation on the ground, foreign fighters are finding it increasingly difficult to cross the Syrian border from Turkey, cutting one of their main routes to join IS.