UPDATE 3-NATO to boost Baltic Sea presence after power, internet cable damage

Updates with NATO reaction in paragraphs 1, 4-5, Finnish president in paragraphs 4, 12-13, Lithuania reaction, power grid operators in paragraphs 16-18

By Essi Lehto and Anna Ringstrom

HELSINKI, Dec 27 (Reuters) - NATO said on Friday it will boost its presence in the Baltic Sea after the suspected sabotage this week of an undersea power cable and four internet lines, while alliance member Estonia launched a naval operation to guard a parallel electricity link.

Finland on Thursday seized a ship carrying Russian oil on suspicion the vessel caused an outage of the Estlink 2 undersea power cable linking it with Estonia and fibre optic lines, and on Friday said it had asked NATO for support.

Baltic Sea nations are on high alert for acts of sabotage after a string of outages of power cables, telecom links and gas pipelines since Russia invaded Ukraine in 2022, although subsea equipment is also subject to malfunction and accidents.

"We have agreed with Estonia, and we have also communicated to NATO Secretary General Mark Rutte, that our wish is to have a stronger NATO presence," Finnish President Alexander Stubb told a press conference.

Rutte said he had discussed with Stubb the Finnish-led investigation, expressing his support.

"NATO will enhance its military presence in the Baltic Sea," Rutte wrote on social media platform X.

The Kremlin said on Friday Finland's seizure of a ship carrying Russian oil was of little concern for the Russian administration. In the past, Moscow has denied involvement in any of the Baltic infrastructure incidents.

Estonia said its navy had deployed to guard the still operational Estlink 1 subsea cable.

"If there is a threat to the critical undersea infrastructure in our region, there will also be a response," Estonian Foreign Minister Margus Tsahkna said on X.

Tsahkna on Thursday said damage to subsea installations in the region has now become so frequent that it was difficult to believe all were caused merely by accident or poor seamanship.

The incidents showed the need to update centuries-old maritime law to explicitly outlaw damage to undersea infrastructure, the country's justice minister told Reuters on Friday.

The 658 megawatt (MW) Estlink 2 outage began at midday local time on Wednesday, leaving only the 358 MW Estlink 1 linking Finland and Estonia, the countries' electricity grid operators have said.

Finnish investigators believe the seized ship - a Cook Island-registered vessel named as the Eagle S - may have caused the damage by dragging its anchor along the seabed, one of several such incidents in recent years.

SHADOW FLEET

The Finnish president said it had been necessary to stop the Eagle S to prevent further destruction.

"Had it kept going anchor on the seabed, more damage would have occurred," Stubb said.

Finland's customs service has said it believed the ship was part of so-called shadow fleet of ageing tankers that seek to evade sanctions on the sale of Russian oil.

But Finnish Prime Minister Petteri Orpo, when asked about the Eagle S, on Thursday said it was too early to say if Russia had played any role in the cable damage.

Finland's Fingrid and Estonia's Elering grid operators expect repairing the Estlink 2 to take months, with an estimated return to service on Aug. 1 of 2025.

The outage could push up electricity prices during the winter months but will not prevent a planned decoupling in February of Estonia, Latvia and Lithuania from the Soviet-era joint power grid with Russia and Belarus, Elering has said.

Lithuania said in a statement on Friday its navy had increased surveillance and patrolling in the Baltic Sea following the incident and that it would support Estonian and Finnish initiatives.

Swedish police continue leading a criminal investigation into the breach last month of two Baltic Sea telecom cables, and have named a Chinese ship travelling from Russia as a possible culprit.

Separately, Finnish and Estonian police continue a probe into last year's damage to the Balticconnector gas pipeline and several telecom cables in which another Chinese vessel arriving from Russia was named.

(Reporting by Essi Lehto in Helsinki and Anna Ringstrom in Stockholm, additional reporting by Andrius Sytas, editing by Terje Solsvik and Tomasz Janowski)

((anna.ringstrom@thomsonreuters.com;))