European leaders fear onslaught of jihadists fleeing from Mosul after Iraq's government and its allies kick ISIS out of the city.

Last year's Paris attacks and the Brussels attacks in March brought heightened awareness that ISIS established an underground network to move jihadis in and out of Europe at will. Thousands of European nationals traveled to Syria and Iraq to wage jihad for ISIS. An estimated 2,500 Europeans still belong to ISIS's fighting force.

"The retaking of (the Islamic State of Iraq and the Levant's) northern Iraq stronghold, Mosul, may lead to the return to Europe of violent (ISIS) fighters," European Union Security Commissioner Julian King told The (London) Telegraph. "This is a very serious threat and we must be prepared to face it."

Iraqi forces, together with Iranian-backed Shiite militia and Kurdish pershmerga, aim to deal a deathblow to ISIS's caliphate in Mosul.

It is a day ISIS anticipated. In an online publication last December called Black Flags From the Islamic State, ISIS vowed to continue its fight.

"If they win this battle, they will capture a lot of weapons, and their soldiers morale will be boosted. Now they will have control over land and will be able to train more people to fight the enemy. If they continue the fight, they will keep winning, but if they start to lose and give up, their leadership will hide in the deserts and mountains again, only to start the: Lone wolf -> Clandestine Cells -> Insurgency -> Army technique, all over again," Black Flags From the Islamic State promised.

Jihadis without a home base pose a direct threat to Europe and menace security officials around the world, warned Raffaello Pantucci, director of the International Security studies at the Royal United Services Institute.

This especially concerns France, which suffered the Paris attacks last November that claimed 130 lives at the hands of ISIS jihadis who fought in Syria. An estimated 400 French nationals are still fighting jihad in warzones.

The number of returnees on watch lists has overstretched European security services, just as ISIS hoped. More than 10 officers try to monitor each returnee around the clock.

"We've had hundreds returned to our country [UK.] Some estimates say it's a thousand. We can't monitor the people that are here. So, it is really important that they sit round the table, because there are potentially 9,000 ISIS jihadists sitting in Mosul at the moment, who are also looking to move across," European Parliament member Janice Atkinson told Russia Today.

The conflict against ISIS is moving into a new, unpredictable phase that has Europe on edge worrying about what comes next.