Finnish authorities said they had boarded and taken control on Thursday of an oil tanker travelling from Russia, on suspicion it had caused the outage on Wednesday of an undersea power cable and three internet lines connecting Finland and Estonia.

The Cook Islands-registered ship, named by authorities as the Eagle S, was boarded by a Finnish coast guard crew which took command in the Baltic Sea and sailed the vessel to Finnish waters, a coast guard official told a press conference.

"From our side we are investigating grave sabotage," said Robin Lardot, Director of the Finnish National Bureau of Investigation, which leads the multi-agency probe.

Director Robin Lardot of the National Bureau of Investigation of Finland (NBI) and Finnish National Police Commissioner Ilkka Koskimaki attend a press conference, amid an investigation of the electricity transmission between Finland and Estonia, through the Estlink 2 connection which was cut on Christmas Day, according to Finnish grid operator Fingrid, in Helsinki, Finland, December 26, 2024. Lehtikuva/ Jussi Nukari via REUTERS

"According to our understanding an anchor of the vessel that is under investigation has caused the damage," he added.

The Finnish customs service said it had seized the vessel's cargo and that the Eagle S was believed to belong to Russia's so-called shadow fleet of ageing tankers that seek to evade sanctions on the sale of Russian oil.

Both the Finnish and the Estonian governments will hold extraordinary meetings later on Thursday to assess the situation, they said in separate statements.

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

Repairing the 170 km (106 miles) Estlink 2 interconnector will take months, and the outage could cause a tense power supply situation during winter, operator Fingrid said in a statement.

Damage to subsea installations in the Baltic Sea has now become so frequent that it is difficult to believe this was caused merely by accident or poor seamanship, Estonian Foreign Minister Margus Tsahkna said in a statement.

"We must understand that damage to submarine infrastructure has become more systematic and thus must be regarded as attacks against our vital structures," Tsahkna said.

The 658 megawatt (MW) Estlink 2 outage began at midday local time on Wednesday, leaving only the 358 MW Estlink 1 in operation between the two countries, operator Fingrid said.

Twelve Western countries on Dec. 16 said they had agreed measures to "disrupt and deter" Russia's so-called shadow fleet of vessels in order to prevent sanctions breaches and increase the cost to Moscow of the war in Ukraine.

"We must be able to prevent the risks posed by ships belonging to the Russian shadow fleet," Finnish President Alexander Stubb said in a post on social media X on Thursday.

Police in Sweden are meanwhile leading an investigation into the breach last month of two Baltic Sea telecom cables, in an incident German Defence Minister Boris Pistorius has said he assumed was caused by sabotage.

Separately, Finnish and Estonian police continue to investigate damage caused last year to the Balticconnector gas pipeline linking Finland and Estonia, as well as several telecom cables, and have said this was likely caused by a ship dragging its anchor.

In 2022 the Russia-to-Germany Nord Stream gas pipelines running along the seabed in the same waters were blown up, in a case still under investigation by Germany.