Nato has said it will enhance its military presence in the Baltic Sea, and Estonia has sent a patrol ship to protect its Estlink1 undersea power cable, after Russia was accused of sabotaging its main power link in the Gulf of Finland.

A ship named as Eagle S is suspected of damaging the Estlink 2 cable and Finnish coast guard crew have boarded the oil tanker and steered it into Finnish waters.

The EU said the Eagle S was part of “Russia’s shadow fleet” and the failure of the undersea cable was the “latest in a series of suspected attacks on critical infrastructure”.

Estonian Defence Minister Hanno Pevkur said the Raju had set sail early on Friday and he believed Finland would join the operation to protect the remaining cable.

He told Estonian public radio that the Raju’s task was “to ensure that nothing happens there and that our critical connection with Finland remains operational”.

The Kremlin has declined to comment on the damage to the cable, describing it as a “very narrow issue” and not an issue for the Russian presidency.

Nato Secretary General Mark Rutte said on social media that he had spoken to Finland’s president, Alexander Stubb, adding that Nato would boost its presence in the Baltic. A further statement by the alliance said only that “Nato remains vigilant and is working to provide further support”.

Finland and Estonia are both Nato members and Estonian Prime Minister Kristen Michal told public TV that, if necessary, they would invoke Article 4 of the Nato Treaty, which involves consultation if any member state feels threatened.

“Our wish would be to receive reinforcements from Nato in the form of a fleet to act as a deterrent,” he was quoted as saying by news agency BNS.

Estonia’s power supply has been dramatically reduced after its 170km (105-mile) Estlink 2 cable was shut down.

In its initial assessment on Thursday, Finland’s Fingrid company said repairs to the cable could last until the end of July 2025.

The damage to Estlink 2 is the third incident in little more than a month in the Baltic Sea.

Last month, two data cables were severed: the Arelion cable between the Swedish island of Gotland and Lithuania on 17 November, and then the C-Lion 1 cable was damaged between the Finnish capital, Helsinki, and the German port of Rostock.

A Chinese ship, the Yi Peng 3, was suspected of dragging its anchor over the cables in a separate act of Russian sabotage.

In October 2023, another Chinese ship ruptured an undersea gas pipeline between Finland and Estonia.

The Yi Peng 3 and Eagle S are both suspected of being part of a so-called shadow fleet of oil tankers that Russia is using to avoid Western sanctions imposed since the full-scale invasion of Ukraine.

The EU said it was working on measures including sanctions to target “Russia’s shadow fleet, which threatens security and the environment”.

After several weeks at anchor in the Kattegat strait between Sweden and Denmark, the Chinese tanker was eventually boarded by authorities from Sweden, Denmark, Germany and Finland, but then set sail last week.

By contrast, Finnish authorities said they had boarded the Cook Islands-registered Eagle S in the early hours of Thursday and it was escorted towards the Finnish coast off Porkkala, across the Gulf of Finland from Tallinn.

“Our patrol vessel travelled to the area and could determine visually that the vessel’s anchor was missing,” said Markku Hassinen, deputy head of the Finnish Border Guard, told a news conference.

The Estonian prime minister sought to reassure Estonians on Thursday that they would continue to have secure power supplies.

The two main power companies, Elering and Eesti Energia, had various reserve and back-up power plants, he told reporters.

However, he added that it was impossible to protect every square metre of the seabed at all times.