Sailors stand on the German attack submarine U-33 before its participation in Dynamic Mongoose, NATO's anti-submarine warfare exercise held annually off the coast of Norway. Steven Beardsley/Stars and Stripes

BERGEN, Norway — Naval forces from 10 NATO nations and Sweden have gathered here to practice detecting and hunting submarines in an annual exercise whose real-world relevance has grown in the past year.

Dubbed Dynamic Mongoose, the training pits submarines from Germany, the U.S., Sweden and Norway against surface ships employing a suite of sonar, sensors and maritime air patrols.

The two-week exercise follows recent reports of foreign vessels in the territorial waters of several European nations, including an incident off Finland’s coast last week and the spotting of a mystery vessel off Sweden last year. Although governments have not publicly blamed any country, they have left little doubt about their suspicion of Russia.

Naval officers speaking to reporters Sunday said the exercise isn’t intended to send a message to any particular country.

“Obviously, we’re aware of the incidents that have happened in some of our partner nations’ waters,” said Rear Adm. Brad Williamson, who commands the Standing NATO Maritime Group 2 ships participating in the exercise. “I think for us what it does is it focuses our efforts and our training here.“

Since Russia’s intervention in Ukraine last year, Sweden has moved closer to the transatlantic alliance, with which it has maintained a close partnership over the years. Some political leaders have even flirted publicly with the idea of one day joining NATO, though no formal steps have been taken in that direction.

Among the exercise’s challenges is an unfamiliar North Sea environment, where cold temperatures and diversity of underground structures and wildlife will affect how submarines maneuver and are detected.

Participant ships work slowly at first, practicing and mastering certain tactics before beginning plugging into drawn-out scenarios, Williamson said. One focus may be protecting an oil tanker or supply ship that might make for an appealing torpedo target.

At their disposal will be maritime air patrols from Norway’s Stavanger Air Base to the south and helicopters embarked aboard their ships, as well as sonar arrays towed behind their vessels.

NATO’s research vessel, the NRV Alliance, will participate in the exercise for the first time, making the journey from its port in La Spezia, Italy, to test unmanned underwater vessels and sensor buoys meant to provide a fixed sonar signal.

New advances are aiding both sides in an old game of cat-and-mouse, said Kevin LePage, a program manager at NATO’s Centre for Maritime Research and Experimentation. “It’s been a technology race,” LePage said. “The sensors are getting better. The submarines are getting better.”

For Cmdr. Kai Nickelsdorf and his crew aboard the German attack submarine U-33, their advantage will be their knowledge of the underwater range. They are vulnerable when they rise to periscope depths, however.

The art of hunting submarines “is the most difficult task in all of naval and NATO tasks,” Nickelsdorf said.

Added to that difficulty is the complex nature of NATO operations, which rely on forces from different nations, cultures and languages to work on a common plain. Williamson, the commander of the naval group, said the only way to do it is in person.

“I’m going up against a thinking opponent who’s going to do his best obviously to carry the day from his side,” he said. “That kind of interaction between the submarine forces and the surface forces in these kinds of waters is really what’s the huge benefit of this exercise.”

Stars and Stripes reporter John Vandiver contributed to this report.