This is one of the reasons why the international police body is preparing to expand a pilot program known as I-Checkit, under which airlines bounce passenger information off Interpol's databases _ in hopes that one day the system could expand to include cruise operators, banks, hotels and other private-sector partners.

Turkey, with its long and often porous border with Syria, has been a major thoroughfare for many of the thousands of foreign fighters seeking to join extremists like the ISIS group, which has captured territory across Iraq and Syria.

Speaking in Monaco, where Interpol is holding its general assembly this week, outgoing chief Ronald Noble confirmed that Turkey was a destination, but declined to identify any others. He also refused to indicate how many people might be involved, but called on countries to step up screening at all transportation hubs _ “airports and, more and more, cruise lines.”

Turkish authorities say they have set up teams to nab suspected foreign fighters in airports and bus stations, and have deported hundreds in recent months.

Pierre St. Hilaire, director of counterterrorism at Interpol, suggested that the Turkish crackdown has shown results in recent months, and so some would-be “extremist” are making alternative travel plans.

“Because they know the airports are monitored more closely now, there's a use of cruise ships to travel to those areas,'' he told the AP on Thursday.

“There is evidence that the individuals, especially in Europe, are traveling mostly to Izmit and other places to engage in this type of activity,'' he said, referring to a Turkish coastal town.

The phenomenon is relatively new, within the past three months or so, said other Interpol officials.

“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,'' Noble said.

“But as we've gathered data, we've realized 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.''

Cruise ships, which often make repeated stops, offer an added benefit by allowing would-be “extremist” to hop off undetected at any number of ports _ making efforts to track them more difficult.

St. Hilaire said it wasn't exactly clear yet how many would-be foreign fighters were traveling by cruise ship to reach Syria, and added that there were other options as well: to avoid passing through airports, some people have driven all the way from their homes in Europe to the Syrian border.

He was quick to caution that Europe is by no means the only or even the main source of foreign fighters for Syria.

“It's a global threat _ 15,000 fighters or more from 81 countries traveling to one specific conflict zone,'' he said, noting that that there are some 300 from China alone. “In order to prevent their travel and identify them, there needs to be greater information-sharing among the region, among national security agencies.''

Elinore Boeke, director of public affairs for the Cruise Lines International Association, the world's largest cruise industry trade association denied security, at least in the U.S., was any more lax than other means of transportation.

“Cruise lines take security as seriously as the airlines, and security procedures are very similar. U.S.-based cruise lines share passenger manifests with U.S. authorities who check against official databases,'' Boeke said in an email.

Many European governments have expressed concern that home-grown “jihadis” who self-radicalize online and then travel to Syria will return home with skills to carry out terror attacks.