Would-be jihadis are booking tickets on cruise ships in an attempt to reach Syria and Iraq without attracting the attention of Turkish security forces.
Interpol has urged screening to be improved at all transport hubs – “airports and, more and more, cruise lines” – to stop fighters reaching Turkey, from where thousands of foreigners seeking to join Islamic State have already crossed into Syria.
The warning came as US aircraft bombed militants from the al-Qaeda-linked Nusra Front and the Ahrar al-Sham brigades in northern Syria.
US officials have said a Nusra Front cell is planning an attack against American interests. Initial reports suggested French-born bomb maker David Drugeon was among those hit and possibly killed in the airstrikes.
Special teams have been set up at Turkish airports and bus stations to intercept foreign jihadis aiming to cross the long and porous border, according to local officials.
Hundreds have been deported in recent months, but the success means militants have begun looking for alternatives ways to travel.
Pierre St Hilaire, director of counterterrorism at Interpol, told AP: “Because they know the airports are monitored more closely now, there’s a use of cruise ships to travel to those areas.
“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.
Cruise ships have only become a concern in the last three months, Interpol officials added.
“Originally, our concern about people on cruise ships – dangerous people on cruise ships – really focused on the classic sort of rapist, burglar or violent criminal,” Ronald Noble, head of the international policing body, said in Monaco.
“But as we’ve gathered data, we’ve realised that there are more and more reports that people are using cruise ships in order to get to launch pads, if you will – sort of closer to the conflict zones – of Syria and Iraq.”