Interpol is warning that would-be jihadist fighters are travelling on cruise ships to try to reach conflict areas in the Middle East.
The international police body said some of those trying to join militant groups in Iraq and Syria had used cruise lines to get to countries including Turkey, the BBC reported.
It said checks to passenger lists should be extended from airlines to cruise operators before the issue became more of a problem.
No figures were put on how many militants had travelled in this way.
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Speaking in Monaco, Interpol's outgoing chief, Ronald Noble, said countries should conduct checks on all passengers using airports "and, more and more, cruise lines".
Turkish authorities said they had deported hundreds of suspected foreign jihadists in recent months after detaining them at airports and bus stations, while Interpol officials said the militants' use of cruise ships had emerged only in the "past three months or so".
A recent United Nations report estimated there were 15,000 foreign jihadists from more than 80 countries fighting with Islamic State (IS) and other extremist groups in Syria and Iraq.