The German navy U-35 is one of the submarines taking part in exercise Playbook Merlin 25 in the Baltic Sea. The five-day NATO exercise is focused on anti-submarine warfare. (NATO)

NAPLES, Italy — NATO allies are strengthening their anti-submarine capabilities this week in the Baltic Sea, a region whose infrastructure network makes it susceptible to Russian mischief threatening Europe’s security.

The U.S., Sweden and Germany are among nine participants in Playbook Merlin, a five-day exercise that uses advanced technology and techniques to ready submarine crews and surface forces for the realities of undersea operations, NATO said in a statement on Tuesday.

“Baltic nations bring an impressive amount of anti-submarine warfare capability to NATO as well as experienced leadership and maritime capacity,” U.S. Rear Adm. Bret Grabbe, commander of NATO submarines, said in the statement.

The exercise demonstrates allied navies’ teamwork “to safeguard sea lines of communication, protect critical infrastructure and hone skills in undersea warfare,” the statement said.

In January, the alliance created a mission dubbed Baltic Sentry after a string of incidents in which undersea telecommunications cables were cut and energy pipeline flows were disrupted. The mission comprises mostly Baltic countries, such as Germany and Finland.

Exercise Playbook Merlin 25 involves submarines from Germany and Sweden, a U.S. Navy maritime patrol and reconnaissance aircraft, and French, German, Dutch and Swedish surface ships and helicopters. The five-day NATO exercise is focused on anti-submarine warfare. (NATO)

U.S. and European intelligence officials the same month said that the incidents likely were the result of maritime accidents rather than part of a broader campaign of Russian hybrid attacks on Europe, The Washington Post reported.

Still, there appears to have been no similar episodes since late January, and in September, Baltic Sentry broadened its scope to include air defense.

That action, which saw the U.S. contribute the destroyer USS Bulkeley and a Navy P-8 patrol and reconnaissance aircraft to the mission, followed repeated unidentified drone incursions into northern European countries.

Bulkeley left in early October, but Navy P-8 aircraft “still periodically support the Baltic Sentry mission,” U.S. 6th Fleet said in a statement Thursday. The departure of the destroyer did not preclude future support by a Navy ship, the fleet added.

On Wednesday, a Navy P-8 was on patrol over the Barents Sea about 50 miles from the coast of northwestern Russia, the open-source intelligence analyst @OSINTdefender said in a post on X.

The aircraft’s location was near one of Russia’s largest and most significant bases for its ballistic missile submarines, about 15 miles from the Norwegian border, according to the post.

This year’s iteration of Playbook Merlin includes a U.S. P-8 and submarines from Sweden and Germany. Surface ships and helicopters from France, Germany, the Netherlands and Sweden also are participating, NATO said.

The exercise is led by NATO’s Allied Maritime Command and hosted by Sweden. NATO did not identify other participants.