The four countries affected by the cuts -- Finland, Germany, Lithuania and Sweden -- have all launched investigations, with Berlin and Swedish police saying they suspected "sabotage".

Finnish telecoms operator Cinia reported Monday that the "C-Lion1 submarine cable" connecting Helsinki and the German port of Rostock had been cut south of Oland island in Swedish waters, some 700 kilometres (435 miles) from Helsinki.

"These kinds of breaks don't happen in these waters without an outside impact," a Cinia spokesperson said.

The foreign ministers of Germany, Annalena Baerbock, and Finland, Elina Valtonen, said it hinted at foul play.

"The fact that such an incident immediately raises suspicions of intentional damage speaks volumes about the volatility of our times," they said in a joint statement late Monday.

"Our European security is not only under threat from Russia's war of aggression against Ukraine, but also from hybrid warfare by malicious actors."

The "Arelion" submarine cable between the Swedish island of Gotland and Lithuania has also been damaged since Sunday, a spokesman for the Lithuanian branch of the operator Telia, Audrius Stasiulaitis, said Tuesday.

"We can confirm that the interruption to internet traffic was not caused by an equipment fault but by material damage to the fibre optic cable," he said.

The Swedish and Lithuanian defence ministers said they were "deeply concerned" by the incident.

"Situations like these must be assessed with the growing threat posed by Russia in our neighbourhood as a backdrop," they said in a joint statement.

Lithuanian Defence Minister Laurynas Kasciunas said the EU "must make best use of its newest sanctions regime for such sabotage of critical infrastructure."

Customers have not been affected as internet traffic has been redirected to other international links, Telia said.

European countries are increasingly using the term "hybrid warfare" to describe actions that they believe are linked to Russia.

- 'Not accidental' -

Sweden's Minister for Civil Defence Carl-Oskar Bohlin told AFP in a written statement it was "crucial to clarify why we currently have two cables in the Baltic Sea that are not working".

German Defence Minister Boris Pistorius also said the two severed cables were a "clear sign that something is going on".

"Nobody believes that these cables were accidentally severed," Pistorius said on the sidelines of a meeting of EU ministers in Brussels.

"We have to say, without knowing exactly who it came from, that this is a hybrid action. We also have to assume, without knowing it yet, that it was sabotage," he said.

Bohlin said Swedish authorities were looking into ships sailing in the area at the time.

"The Swedish armed forces and coast guard have picked up ship movements that coincide in time and space with the interruptions that occurred," he told television TV4.

Swedish public broadcaster SVT and several Finnish media meanwhile reported that two Danish navy ships shadowed a Chinese cargo vessel, the Yi Peng 3, as it sailed out of the Baltic Sea early Tuesday after the cables were severed.

Citing unidentified sources, they said several countries' authorities considered the vessel to be of interest in the investigation.

- Rising tensions in the Baltic -

Since Russia's full-scale invasion of Ukraine in February 2022, there have been repeated cases highlighting the mounting tensions in the Baltic region.

In September 2022, a series of underwater blasts ruptured the Nord Stream pipelines that carried Russian gas to Europe.

In October 2023, an undersea gas pipeline between Finland and Estonia had to be closed after it was damaged by the anchor of a Chinese cargo ship.

Finland has stepped up surveillance of incidents in the Baltic Sea since the outbreak of war between neighbouring Russia and Ukraine.

Last month, NATO opened a new naval base in Rostock to coordinate the forces of the alliance's members in the Baltic Sea.

Russia summoned the German ambassador to Moscow after the inauguration to protest against the new naval command centre.

Moscow called the centre a "blatant breach" of the treaty on the reunification of Germany in 1990 that said no foreign armed forces would be deployed in the area, a claim Berlin denied.

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TELIA