Europe must brace itself for a new wave of Islamic State fighters following the offensive on Mosul by Iraqi and coalition forces, an EU Commissioner has warned.

“The retaking of the IS’s northern Iraq territory, Mosul, may lead to the return to Europe of violent IS fighters,” the EU’s security commissioner, Julian King, told Die Welt. “This is a very serious threat and we must be prepared to face it.”

Approximately 2,500 EU nationals are known to be fighting alongside Islamic State in the Mosul area. King said that even if only a handful returned it would pose a “serious threat that we must prepare ourselves for”.

King has called for stronger anti-terrorism measures, including increased security at Europe’s borders. “Currently, one can bypass the passport controls at the EU’s external borders too easily by providing false documents,” he said.

He added: “We should also consider the security standards of all birth and marriage certificates – the documents upon which travel documents are obtained. Having tamper-proof passports is useless if it has been issued on incorrect information. We must also discuss how national identity cards can be made as secure as passports are.”

Residents of Mosul have reported that dozens of Islamic State fighters have fled in recent weeks, and that more than 50 departed following the announcement of the offensive on Monday morning, The Telegraph has reported.

“They wait until night and leave by car. They are going to al-Ba’aj to the west of Mosul, then on to the Iraq-Syria border, where they continue to Syria and Turkey,” a 35-year-old who gave his name as Ahmed, said.

It is thought that Islamic State has around 4,000-8,000 fighters remaining within Mosul, but that the group is recruiting civilians, including children, for what has been called the “mother of all battles”. Some 25,000 Iraqi troops are involved in the operation to retake Mosul, along with Kurdish Peshmerga, U.S. forces, German reconnaissance planes, Turkish soldiers, and Shi’ite militiamen.

Last week it was reported that the U.S., which is backing Iraqi troops in the region, may not entirely encircle the city so as to offer fighters a route out via the western side of the city, thus avoiding greater civilian casualties.

Should that happen, the jihadis, with no major territory left in Iraq, will likely move either into Syria or into other regions the group is hoping to target, including Europe.

Analysts say that even if the group loses all of its territory it will not mark the end of Islamic State – and that the development could have potentially devastating consequences for the West.

Chris Phillips, managing director of counter-terrorism consultancy Ippso, explained that the group is “entering a new phase”, and that losing their “caliphate” in Syria and Iraq “would force them into more guerrilla or terrorist actions”.

With the retaking of Mosul, “I think we will see a growth of terrorist attacks across North Africa and the West,” he said, adding that fighters would likely use the routes used by so-called refugees to access Europe.

Meanwhile, Commission President Jean-Claude Juncker has spoken of the need to close “terrorist loopholes” by implementing a new European Travel Information and Authorisation System (ETIAS). He wants work on the first draft of such a system to be ready by November.

However, a spokesperson from the German interior ministry told EurActiv that the purpose of ETIAS is not necessarily compatible with Juncker’s quest to close the loopholes.

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