Several telecom and power cables have been severed with experts and politicians accusing Russia of orchestrating a hybrid war against the West as the two sides square off over Ukraine.

The "Baltic Sentry" mission would involve "frigates and maritime patrol aircraft" among other assets, NATO chief Mark Rutte said at a regional meeting in Finland's capital Helsinki on Tuesday.

But he declined to give details on the number of vessels "because that might differ from one week to another" and he did not want to make "the enemy any wiser than he or she is already".

NATO was also tight-lipped on the duration, saying in a statement the operation would continue "for an undisclosed amount of time".

The suspected sabotage has been blamed on a "shadow fleet" of vessels -- often ageing and operating under opaque ownership -- that carry Russian crude oil and petroleum products, embargoed since the invasion of Ukraine.

"Investigations of all of these cases are still ongoing, but there is reason for grave concern," Rutte said.

He said protecting undersea infrastructure was of "utmost importance" not only for energy supplies but also for internet traffic.

- 'Strategic signal' -

Leaders of NATO's Baltic countries said in a statement after the Helsinki meeting that the shadow fleet "poses a particular threat to the maritime and environmental security in the Baltic Sea region and globally".

They said the fleet "significantly supports funding of Russia's illegal war of aggression against Ukraine".

Finland's President Alexander Stubb said foreign ministries from the Baltic Sea NATO states would set up a group of legal experts to assess what they could do without affecting freedom of navigation.

NATO said in late December it would increase its presence in the region but had not announced an operation.

Iro Sarkka, a researcher from the Finnish Institute of International Affairs, told AFP that NATO had been pushed into action by the Russian shadow fleet.

A comprehensive operation would serve as a "deterrent and a strategic signal" that NATO was prepared to act, according to Sarkka.

- Repeated incidents -

Tensions have mounted around the Baltic Sea since Russia's invasion of Ukraine in February 2022.

A series of underwater blasts ruptured the Nord Stream pipelines that carried Russian gas to Europe in September 2022, the cause of which has yet to be determined.

In October 2023, an undersea gas pipeline between Finland and Estonia was shut down after it was damaged by the anchor of a Chinese cargo ship.

Two telecom cables in Swedish waters were severed on November 17-18 last year.

And weeks later, on December 25, the Estlink 2 electricity cable and four telecom cables linking Finland and Estonia were damaged.

Investigators suspect the cables were damaged by the anchor of the Eagle S, a Cook Island-flagged oil tanker believed to be part of the "shadow fleet".

Finnish police seized the Eagle S on December 28 as part of a criminal investigation.

Finnish authorities last week deemed the ship unseaworthy, barred it from sailing and have banned eight crew members from leaving the country while police carry out a probe.