OSLO (Reuters) - Finnish police said on Sunday they had found tracks that drag on for dozens of kilometres along the bottom of the Baltic Sea where a tanker carrying Russian oil is suspected of breaking a power line and four telecoms cables with its anchor.

The Cook Islands-registered Eagle S was boarded by Finnish police and coast guard officials on Thursday and sailed into Finnish waters where the crew of the impounded tanker is being questioned.

Baltic Sea nations have been on high alert after a string of outages of power cables, telecom links and gas pipelines since Russia invaded Ukraine in 2022. NATO said on Friday it would boost its presence in the region.

A break in the 658 megawatt (MW) Estlink 2 power cable between Finland and Estonia occurred at midday on Wednesday, leaving only the 358 MW Estlink 1 linking the two countries, grid operators said. They said Estlink 2 might not be back in service before August.

Finnish police suspect the Eagle S caused the damage by dragging its anchor along the seabed.

Investigators have identified a "dragging track" but have yet to find a missing anchor, Sami Paila, tactical leader and detective chief inspector of Finland's National Bureau of Investigation, said in a statement.

"The track is dozens of kilometres in length," Paila said.

Photos taken of the Eagle S on Friday showed the vessel missing its port side anchor.

Finland's customs service believes the ship is part of a "shadow fleet" of ageing tankers being used to evade sanctions on exports of Russian oil.

The Kremlin said on Friday that Finland's seizure of the ship was of little concern to it.

Russia has denied involvement in any of the previous Baltic infrastructure damage incidents.

(Reporting by Terje Solsvik; editing by Jason Neely)