German Defence Minister Boris Pistorius has said damage to two undersea cables in the Baltic Sea looks like an act of sabotage and a “hybrid action”, without knowing who is to blame.

A 1,170km (730-mile) telecommunications cable between Finland and Germany was severed in the early hours of Monday, while a 218km internet link between Lithuania and Sweden’s Gotland Island stopped working on Sunday.

The incidents came at a time of heightened tension with Russia and Pistorius said “nobody believes that these cables were cut accidentally”.

A series of incidents involving Baltic pipelines have heightened fears of sabotage since Russia’s full-scale invasion of Ukraine.

Germany and Finland have both said they are “deeply concerned” by the severing of the C-Lion1 cable communications cable, adding that Europe’s security is threatened by Russia’s war, “but also from hybrid warfare by malicious actors”.

Finnish telecoms and cyber security firm Cinia said its cable may have been severed “by an outside force”. “These kinds of breaks don’t happen in these waters without an outside impact,” a spokesperson told local media.

Cinia’s chief executive said the damage had taken place close to Sweden’s Oland Island and could take five to 15 days to repair.

The two cables intersect in the Baltic although the damage is thought to have taken place elsewhere. Arelion, the company that operates the line to Lithuania, has not said where its cable was cut but expected the repair to take a couple of weeks.

Swedish civil defence minister Carl-Oskar Bohlin said it was “absolutely central” to find out why two cables were not working. About a fifth of Lithuania’s internet capacity has been reduced, although consumers are understood not to be affected.

Arelion spokesman Martin Sjogren told the BBC that cables in the Baltic did get damaged every now and then.

“Fishing vessels accidentally damage cables with anchors,” he said. “The timing is odd of course but we haven’t been able to examine it so we don’t know what caused it.”

Samuli Bergstrom, a Finnish government cybersecurity expert, said the failure of the cable from Finland to Germany had not affected internet traffic as other cable routes were available.

The biggest act of sabotage since Russia’s 2022 invasion of Ukraine took place in the Baltic in the same year.

German prosecutors are still investigating the explosion of Nord Stream gas pipelines between Russia and Germany.

There have been conspiracy theories around that attack, with unconfirmed rumours that either the Ukrainian, Russian or US government was behind it.

In October 2023 a natural gas pipeline between Finland and Estonia was severely damaged.

Finnish officials later said the incident had been caused by a Chinese container ship dragging its anchor.