STOCKHOLM: Eight NATO countries have started five days of anti-submarine exercises in the Baltic sea and a top commander called the ultra-silent underwater vessels a constant “threat.”

Based in Sweden from Monday to Friday, the submarine-hunting drills also involve Britain, Canada, Denmark, Finland, France, Germany and the Netherlands. The Baltic has become a key venue for the so-called war of “hybrid” tactics that Western nations blame on Russia.

Several thousand naval forces, submarines from Germany and Sweden, surface vessels from several countries and air patrols will take part in the staged hunts for submarines, the alliance said.

NATO has stepped up its naval presence in the Baltic, where, military experts say, submarine signals — the key to tracking vessels — can be more easily hidden in its shallow waters crisscrossed by intense shipping traffic.

“The submarine is the only technology at this point that can dominate that undersea consistently,” Rear Admiral John Craddock, the NATO commander for submarines, told AFP.

“And so since that submarine can still dominate from the undersea, you have to be able to find them,” he added, highlighting that detecting submarines was “a perishable skill.”

“We’ve got to practice it all the time because until we can make the oceans truly transparent, submarines are going to be a threat.”

Rear Admiral Nicolas Molitor of the French navy said that the Baltic, with its many undersea cables and pipelines, faces other threats.

“There is also the threat from vehicles or robots that can operate under the sea,” Molitor told AFP on the sidelines of the exercises. He said that surface vessels that drag their anchor on the sea bed could damage “essential” infrastructure.