The hulking grey vessel -- the flagship of one of NATO's four permanent multinational fleets known as standing maritime groups -- is docked in southeast London for a port visit, after months of conducting training exercises, including as part of the alliance's Baltic Sentry patrol mission.

Baltic Sentry was pulled together in January to protect underwater infrastructure -- thousands of kilometres of cables critical for global internet traffic -- after five cables were severed in the Gulf of Finland in December 2024.

The Baltic Sea has become a flashpoint for tensions between Russia and NATO since Moscow's 2022 invasion of Ukraine, worsened by its Baltic neighbours Finland and Sweden's subsequent decision to join the alliance.

Military experts and European leaders say Russia has ramped up its "hybrid war" in the strategic region -- now bordered entirely by NATO members, with the exception of Russia -- through airspace incursions and suspected sabotage of undersea cables.

"But, since we started we haven't seen any malign activity. There were quite a few incidents before," said Craig Raeburn, the Chief of Staff of the NATO standing group that Johan de Witt belongs to.

"We have deterred that activity," Raeburn said from the command deck of the ship.

But according to the fleet's Commander Arjen S. Warnaar, protecting undersea infrastructure in the Baltic is also a "legal issue" and those who damage it should be "held accountable".

Crewmembers of the Cook Islands-registered Eagle S oil tanker, thought to be part of Russia's "shadow fleet" of ships with dubious ownership used to skirt western sanctions, were accused of dragging their anchor across the seabed in the Gulf of Finland, damaging the cables.

Earlier this month, a Helsinki court dismissed the case for being beyond Finnish jurisdiction, which prosecutors said they will appeal.

"You can prove who did it, but can you prove if he did that on purpose? That's much harder," said Warnaar.

NATO commanders say defence capabilities in the region have been bolstered by Finland and Sweden's expertise in protecting underwater infrastructure as well as in areas such as anti-submarine warfare and managing drone incursions.

"There's a lot of effort done by the Baltic states," said Warnaar. "But we're there. We're looking. They know we're looking. And that has an effect."