Until recently Turkey had been the main entry point for prospective fighters for the so-called Islamic State. But increased security, which has resulted in hundreds of deportations, has meant that the border is no longer as porous as it was.

Jihadists are now looking at less conventional ways of joining the fighting in Iraq and Syria, including booking trips on cruise liners, Interpol claim.

The organisation's director of counter-terrorism Pierre St Hilaire spoke to AP about the problem:

"There is evidence that the individuals, especially in Europe, are travelling mostly to [the Turkish coastal town of] Izmit and other places to engage in this type of activity."

He said that the use of cruise ships has developed over the last three months.

Though cruise ships do employ security personnel to check identities, St Hilaire believes more information sharing is required between intelligence agencies to combat this growing threat.

An estimated 15,000 foreign fighters from over 80 countries have so far joined IS, whose territory contains about six million people.