On December 25, the Estlink 2 electricity cable and four telecom cables linking Finland and Estonia were damaged, just weeks after other Baltic Sea cables were severed in similar incidents that experts and politicians say are part of hybrid war actions orchestrated by Russia.

NATO announced late last month that it would strengthen its military presence in the Baltic Sea in response to the cable cuts.

Finland's Foreign Minister Elina Valtonen said Friday that the US-led military alliance was sending two vessels to surveil the Russian "shadow fleet" and critical undersea infrastructure.

"NATO is sending two vessels and we are also increasing other activities and presence in the area," Valtonen told a press conference.

On Monday, the British-led Joint Expeditionary Force (JEF), which comprises Nordic and Baltic states, as well as the Netherlands, announced it would also increase its surveillance of undersea infrastructure in the Baltic Sea in response.

Suspicion over the Christmas Day incident has fallen on the Eagle S, a Cook Island-flagged oil tanker believed to be part of the "shadow fleet".

Russia's "shadow fleet" consists of ships that carry crude oil and petroleum products embargoed over Moscow's invasion of Ukraine.

The often ageing vessels operate under opaque ownership or without proper insurance.

Finnish police seized the vessel on December 28 as part of a criminal investigation into the suspected sabotage.

The ship is anchored in Porvoo, east of Helsinki. Finnish authorities have banned eight crew members from leaving the country while the police probe is carried out.

Investigators suspect the cables were damaged by the tanker's anchor being dragged over them.

On Tuesday, police said the Eagle S anchor had been recovered from the seabed.

"Finland has long been concerned about the risks to environmental and maritime safety posed by the shadow fleet used by Russia," Valtonen said.

"It is clear that the shadow fleet is also a threat to critical underwater infrastructure."

Finland's President Alexander Stubb and Estonian Prime Minister Kristen Michal will co-host a summit of NATO countries bordering the Baltic Sea in Helsinki next week to discuss security in the region.

Valtonen said she was expecting concrete proposals from the upcoming meeting that will be attended by NATO Secretary General Mark Rutte, the leaders of Denmark, Germany, Latvia, Lithuania, Poland and Sweden, and the executive vice-president of the European Commission Henna Virkkunen.