ON THE BALTIC SEA — The crew of the Norwegian minehunter gave the black speedboat skimming the waves toward it one last chance to turn back before opening fire with machine guns mounted on the deck. 

From the bridge of the KNM Hinnøy, officers sounded an air horn and shouted warnings over a loudspeaker. But when the boat kept coming, they started shooting.

This was just an exercise, intended to keep the crew of the Hinnøy sharp as they focused on their real mission: confronting a quiet and stealthy form of Russian aggression. 

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Even as Presidents Donald Trump and Vladimir Putin negotiate the fate of the war in Ukraine — and express hope for better relations between Washington and Moscow — America’s NATO allies say there has been no letup in Russian sabotage operations across Europe. 

One of the Kremlin’s newest threats, according to NATO, is targeting undersea cables carrying vast quantities of data and power beneath the Baltic Sea to countries in northern Europe. 

Defending the cables — some of which are no thicker than garden hoses — is the task of a NATO naval patrol known as Baltic Sentry which started in January. NBC News joined the flotilla last week as they sailed the icy sea off the coast of Poland, playing cat-and-mouse with suspected Russian saboteurs. 

It’s a game where the rules are murky and the geopolitical stakes are high. 

“We are the safety cameras of the Baltic Sea region, and anything that is undertaken unlawfully against critical underwater infrastructure will be seen,” said Cmdr. Erik Kockx, a Belgian naval officer leading the task force.

As he spoke, crew members of the Dutch ship HNLMS Luymes were scanning the horizon with binoculars while computer screens displayed sonar imagery of the cables hundreds of feet below alongside wrecks from World War II. 

NATO has accused Russia of waging “hybrid warfare” on both land and sea — covert acts of sabotage big enough to damage European economies but small enough to avoid triggering a Western military response. “It is really state-sponsored sabotage, and in some cases even state-sponsored terrorism,” said Mark Rutte, the organization’s secretary general, at a summit last month.

In the Baltic, analysts say the threat is not from clearly flagged Russian military vessels but from the “shadow fleet,” merchant vessels that are secretly acting on orders of the Kremlin. 

One suspected member of the shadow fleet, according to Finland’s government, is the Eagle S, an aging oil tanker that set sail near St. Petersburg on Christmas Day on a lumbering course through the Baltic. 

By 1 p.m. (7 a.m. ET) the ship had crossed over the Estlink-2 power cable connecting Finland to Estonia, according to tracking data provided by MarineTraffic, a maritime analytics firm. 

Shortly afterward, authorities detected damage to the Estlink, along with three other cables on the Eagle S’s route.

When Finnish commandos boarded the tanker from a helicopter they discovered an immediate cause for suspicion: The ship’s anchor was missing. Investigators said they believed it broke off as it was dragged along the seabed — slicing through cables as it went. 

The Kremlin had denied any involvement in the Eagle S incident or several other recent cases of damage to undersea infrastructure. Finnish prosecutors have barred three members of the ship’s crew from leaving the country as they weigh bringing criminal charges. 

“The events in the Baltic Sea are connected to and must be viewed within the same prism of that escalating sabotage campaign by Russia across Europe,” Matthew Mooney, director of the threat research division at the U.S. cybersecurity firm Recorded Future, said in an interview.