The European Union and NATO said they were standing by Finland and Estonia as Finnish authorities launched an investigation into the “sabotage” of an undersea power cable connecting the two countries by a Russian-sourced tanker that caused the Christmas Day sudden blackout.

The Eagle S tanker is believed to be part of a “ghost fleet”. The Cook Islands-flagged vessel was carrying unleaded petrol, which it had loaded from a Russian port, Finnish Customs director general Sami Rakshit told a news conference.

In addition to the power transmission cable, submarine telecommunications cables were also reportedly damaged.

The Finnish authorities seized the ship and took it into Finnish territorial waters.

The European Union said it was cooperating with Finnish authorities in the investigation and threatened new sanctions against Russian ships.

“We strongly condemn any deliberate destruction of Europe’s vital infrastructure. The suspect ship is part of Russia’s ghost fleet, which threatens security and the environment while funding Russia’s war budget. We will propose further measures, including sanctions, to target this fleet,” the European Commission and the head of European diplomacy, Kaya Kalas, underlined in a joint statement.

“We are following the investigations of Estonia and Finland and are ready to provide further support,” NATO Secretary General Mark Rutte said in a post on X.

“We are in close coordination with our allies and stand ready to support their investigations,” said a spokesman for the US National Security Council, adding that the incident underscored the need for closer international cooperation to safeguard vital undersea infrastructure.

Finnish President Alexander Stubb has called for the “threats” posed by Russia’s ghost fleet to be “eliminated”.

The country’s Estonian foreign ministry announced that damage to underwater infrastructure “should be considered attacks against our critical infrastructure”. He also noted that the “ghost fleet” not only violates sanctions imposed on Russia but also poses a security threat in the Baltic Sea and “we cannot just sit back and watch.”

“Damage to critical undersea infrastructure has become so common that it is hard to believe that these are accidents or just poor seamaneuvers,” said Estonian Foreign Minister Margus Tsachna.

Lithuanian Foreign Minister Kestutis Badris stressed that the increasing number of incidents in the Baltic Sea should be a stern and urgent warning to NATO and the European Union to significantly strengthen the protection of undersea infrastructure in the region.

Twelve Western countries on December 16 said they had agreed to measures to “disrupt and deter” Russia’s so-called “ghost fleet” to avoid sanctions violations and increase the cost to Moscow of its war in Ukraine.

The Baltic countries are on high alert for possible sabotage due to the disasters that have occurred since 2022 in electricity transmission cables, telecommunications links and natural gas pipelines.