Finnish authorities have seized a ship carrying Russian oil in the Baltic Sea on suspicion it caused the outage of an undersea power cable connecting Finland and Estonia a day earlier, and that it also damaged or broke four internet lines.

The Cook Islands-registered ship, named by authorities as the Eagle S, was boarded by a Finnish coast guard crew on Thursday, that took command and sailed the vessel to Finnish waters, a coast guard official said at a press conference.

"According to our understanding, an anchor of the vessel that is under investigation has caused the damage," said Robin Lardot, director of the Finnish National Bureau of Investigation.

Two fibre-optic cables owned by Finnish operator Elisa, linking Finland and Estonia, were broken while a third link between the two countries owned by China's Citic was damaged, Finnish transport and communications agency Traficom said.

A fourth internet cable running between Finland and Germany and belonging to Finnish group Cinia was also believed to have been severed, the agency said.

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

At a press conference following the meeting, Estonian officials agreed with the Finnish authorities' suspicion that the Cook Islands-flagged tanker Eagle S "intentionally" struck the offshore cable.

Repairing the 170-km Estlink 2 interconnector will take months, and the outage raised the risk of a strained power supply during the winter, operator Fingrid said in a statement. (Agencies)