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, so some would-be jihadis 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 AP yesterday.
"There is evidence that the individuals, especially in Europe, are travelling mostly to Izmit [a Turkish coastal town] and other places to engage in this type of activity," he said.
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 realised 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 jihadis to hop off undetected at any number of ports - making efforts to track them difficult.
St Hilaire said it wasn't exactly clear yet how many would-be foreign fighters were travelling by cruise ship to reach Syria. 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 travelling to one specific conflict zone," he said, noting that there are about 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, public affairs director for the Cruise Lines International Association, the world's largest cruise industry trade association denied security, at least in the US, was any more lax than other means of transportation.
"Cruise lines take security as seriously as the airlines, and security procedures are very similar. US-based cruise lines share passenger manifests with US authorities who check against official databases," Boeke said.
Many European Governments have expressed concern that home-grown jihadis who self-radicalise online and then travel to Syria will return home with skills to carry out terror attacks. Frenchman Mehdi Nemmouche, who allegedly spent a year in Syria and fought with Isis, is the chief suspect in a May attack in Brussels that killed four people.
US strikes hit rebel alliance
The US has broadened its air campaign in Syria, hitting a faction once seen as part of a rebel alliance neither part of al-Qaeda nor the Islamic State.
The attack on Ahrar al-Sham, an Islamist group which has been backed by Qatar and which has been allied to a number of mainstream rebel brigades, is likely to complicate Syria's war.
Rebel groups reacted angrily to the bombing, as they did to a separate attack against the al-Qaeda-linked Jabhat al-Nusra group, which is reported to have killed civilians, including at least two children. Activists in the border town of Harem posted footage of the corpse of a young boy amid rubble. A member of a foreign aid organisation, who was in the town at the time of the air strike, confirmed to the Daily Telegraph that it had caused civilian casualties.
Rebel groups expected, though in many cases opposed, US bombing raids against Isis. But they have been angered by the strikes against Jabhat and Ahrar, saying they are primarily fighting the Assad regime, and that the attacks are, therefore, helping his cause.
