Germany cites Russia's war against Ukraine and the threat of "hybrid warfare" - as Finnish authorities say it could take two weeks to fix the outage.
An undersea fibre optic cable between Germany and Finland has stopped working and might have been deliberately cut by an unknown party, according to authorities.
A physical inspection has not yet been done but the abrupt nature suggests it was completely severed by an outside force, said chief executive Ari-Jussi Knaapila.
Germany and Finland's foreign ministers said they were "deeply concerned" and it "immediately raises suspicions of intentional damage".
A joint statement said: "Our European security is not only under threat from Russia's war of aggression against Ukraine, but also from hybrid warfare by malicious actors.
He said the damage occurred near the southern tip of Sweden's Oland island and that Cinia was working with authorities to investigate.