Cruise ships could be unwittingly transporting jihadist fighters to the Middle East, Interpol has warned.
The international crime prevention organisation said fighters may turning to the seas to reach conflict zones as security on airlines continues to tighten.
Interpol warned checks on cruise passengers should be tightened, with ports in Turkey singled out as a cause for particular concern.
However, industry body CLIA insisted that cruise lines also have stringent security processes in place to confirm the identities of passengers.
“Cruise lines work closely with authorities around the world and have stringent security processes designed to provide for the safety of passengers and crew,” it said in a statement.
“Cruise lines employ full-time security professionals, many of whom are former law enforcement officers, who are in regular contact with local and international authorities.”
Interpol did not disclose a figure but said in the last three months it has emerged that militants are using cruise ships to reach conflict areas in the Middle East.
The Turkish coastal town of Izmit was cited as a possible entry point for European fighters seeking to join conflicts in Iraq and Syria.
A recent United Nations report estimated there were 15,000 foreign jihadists from more than 80 countries fighting with Islamic State and other extremist groups in the Middle East.