The team is part of an international mission enforcing United Nations sanctions on Pyongyang, flying surveillance trips from the US Futenma base in Japan's Okinawa.

"The UN sends us information on vessels suspected of illicit behaviour," and then a flight plan is drawn up, explained Guillaume, the lieutenant commander who leads the team and can only be identified by his first name.

On board their Falcon 200 aircraft, the team of six uses radar as well as ships' AIS -- the automatic identification system that transmits information including a vessel's identity and its route.

But their best tool remains visual observation: two crew members sit next to the plane's windows, scanning the ocean surface with binoculars and taking photographs.

"We're the eyes of the United Nations in the area," said Guillaume.

Suddenly, the team is on alert: a ship has been spotted with its AIS deactivated, a move considered suspect behaviour.

The pilots reroute the plane, making two passes at an altitude of less than 150 metres (500 feet), looking for the boat's name on its stern and trying to establish radio contact with its crew.

Briefing the team before the flight, Guillaume stressed the importance of a "cordial and professional response" in instances of suspicious behaviour.

"In the context of the international tensions in the area, the goal is to avoid poisoning the situation. We have to be firm but courteous."

- 'France's credibility' -

A little farther away, the team spots two stationary ships, hull to hull.

A first pass reinforces suspicions: pumping pipes connect the larger of the ships, whose waterline indicates it is fully loaded, to the smaller one, a merchant ship.

The latter is "ideal for carrying contraband, but could also simply be refuelling fishermen", a team member explains.

When contact is established with the larger boat, the crew claims no knowledge of why the smaller vessel is attached.

The French team gathers as much information as possible on the vessels to send to the UN, which will investigate for violations of Resolutions 2375 and 2397 limiting the sale, supply and transfer of natural gas and petroleum to North Korea.

If a violation is found, a case could move forward against the ships and their owners.

The team arrived in Japan from their base in French Polynesia in mid-October.

French forces have participated in the surveillance missions regularly since 2018, alongside eight other countries and under the supervision of the Enforcement Coordination Cell responsible for implementing UN resolutions.

For France, the missions are also a way to increase its profile in the region, after the 2019 unveiling of its Indo-Pacific defence strategy.

France has 1.6 million citizens and a vast exclusive economic zone of nine million square kilometres in the region, which has particular strategic value given Beijing's growing territorial ambitions and US-China tensions.

And its mission under the UN aegis also illustrates "its interest in the region beyond its Indo-Pacific priorities," said Hugo Decis, a research analyst at the International Institute for Strategic Studies in London.

It confirms "the capacity of French forces to work with their partners and allies and in that sense contributes to France's credibility as a power, even if a secondary one, in the Pacific."