Europol chief warns 5,000 ISIS terrorists may be in Europe

Up to 5,000 Islamic State (ISIS) terrorists may have returned to Europe after being trained in terror training camps in Syrian and other war zones as the continent faces its highest terror threat in over a decade, the head of EU's police agency has warned.

Rob Wainwright, director of Europol - the European Union- wide law enforcement agency, said these trained terrorists may be plotting further attacks like those in Paris last November which claimed 130 lives.

"Europe is currently facing its biggest terror threat in more than a decade," he told Germany's 'Neue Osnabrucker Zeitung' newspaper.

"One can expect Islamic State (ISIS) or other religious terror groups to stage an attack somewhere in Europe with the aim of achieving mass casualties among the civilian population. Additionally, there is the risk of individual attackers - this has not reduced," he said.

Europol estimates that there are between 3,000 and 5,000 Europeans who have travelled to terrorist training camps abroad, including those run by ISIS in its territories in Iraq and Syria, and returned home, the report said.

"The growing number of these foreign fighters presents new challenges for EU countries," Wainwright added.

But he played down fears that terrorists are using the European migrant crisis to infiltrate the continent as asylum seekers at refugee camps.

"There is no concrete evidence terrorists are systematically using the flow of refugees to infiltrate Europe," he said.

However, Austrian prosecutors this week have confirmed a 34-year-old Pakistani suspect, who has not been named, was arrested along with a 28-year-old Algerian at a refugee shelter in the country on December 10 in connection with the serial terror attacks in Paris in November, 2015.

Their arrest came after two other suspects were taken into custody.

An Austrian prosecutors' statement said: "It can be assumed that all four are members of ISIS."

Last month, the Europol in a report had warned that "ISIS is preparing more terrorist attacks, including more 'Mumbai- style' attacks, to be executed in member states of the EU, and in France in particular".