The British head of Europol has warned that as many as 5,000 ISIS-trained jihadists are wandering free in Europe.

Rob Wainwright, chief of the EU's police agency Europol, said the agency believed between 3,000 and 5,000 jihadists have been able to slip back into Europe after training with ISIS in the Middle East.

‘Europe is currently facing the highest terror threat in more than 10 years,’ Mr Wainwright told Germany’s Neue Osnabrucker Zeitung newspaper.

Threat: Head of Europol, Rob Wainwright (pictured), has warned that the agency expects ISIS-trained jihadists to 'stage an attack somewhere in Europe with the aim of achieving mass casualties among the civilian population'

‘We can expect [ISIS] or other religious terror groups to stage an attack somewhere in Europe with the aim of achieving mass casualties among the civilian population.’

He added that the increasing number ‘presents EU member states with completely new challenges’.

But he insisted that claims terrorists are using the migrant crisis to sneak into Europe disguised as asylum-seekers have been exaggerated.

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

Warning: Rob Wainwright, chief of the EU's police agency Europol, said the agency believed between 3,000 and 5,000 jihadists have been able to slip back into Europe after training with ISIS in the Middle East. Pictured, wounded people being evacuated outside the scene of a hostage situation at the Bataclan theatre in Paris

Crisis: Mr Wainwright insisted that claims terrorists are using the migrant crisis to sneak into Europe disguised as asylum-seekers have been exaggerated. Pictured, eefugees and migrants arrive into the port of Mytilene, of the Greek island of Lesbos

It comes as Austrian prosecutors revealed that they are investigating four people in custody for possible links to the Paris attacks.

Calling the investigations ‘highly complex’, Salzburg prosecutors said that all four – two more than previously confirmed – are believed to belong to ISIS, which claimed responsibility for the attacks.

On the run: Salah Abdeslam is wanted by police in connection with the terror attacks in Paris

The officials said that in addition to an Algerian and a Pakistani, whose arrest on December 10 was announced several days later, two others have been detained since December 18.

The first two, aged 28 and 34, were believed to have been in the same migrant boat travelling to Greece as two men involved in the Paris atrocities that killed 130 people, prosecutors said in a statement.

While those involved in the attacks were able to travel onwards, the pair now in Austrian hands were held up by Greek authorities for 25 days because they were carrying fake Syrian passports.

They then arrived in Salzburg in western Austria at the end of November - after the Paris killings - and Austrian police arrested them at a centre for migrants on December 10.

Eight days later, the other two, a 25-year-old Moroccan and a 40-year-old Algerian, were arrested 'because of indications of close contact with both the first two suspects', prosecutors said.

The statement stressed that contrary to some media reports, the men had not confessed to planning any attacks.

Prosecutors added that two other men, aged 22 and 28, who said they were migrants were arrested in Salzburg on September 17 and October 10, after recounting that they had fought for ISIS.