Sweden said on Tuesday the wreck of a submarine found off its coast was probably a Tsarist-era Russian vessel that collided with a Swedish ship about a century ago.

"We are most likely talking about the Russian submarine the Som (Catfish) which sank after a collision with a Swedish vessel in 1916 during World War I and before the Russian revolution," the Swedish Armed Forces said.

Speculation had been swirling about the origins of the vessel after Swedish divers announced Monday that a submarine had been found about 1.5 nautical miles off the coast of central Sweden.

The announcement came nine months after a high-profile hunt for a mystery submarine in Swedish waters, which some suspected was a modern Russian vessel.

The Swedish military said pictures of the wreck taken by the divers confirmed its own analysis and that it did not think a full technical analysis was necessary.

Experts identified it as an Imperial Russian Navy sub that sank with an 18-member crew in May 1916 after a collision with a Swedish vessel.

- 'Immortalise the memory' -

 

Swedish newspaper Dagens Nyhter said it was a submarine built for the Imperial Russian Navy in Vladivostok in 1904 and integrated into the naval fleet in the Baltic Sea in 1915.

"Judging by the pictures, it is the Som," Konstanin Bogdanov, head of a state-backed team of wreck divers in Russia, told AFP, referring to what appears to be Cyrillic lettering on the submarine's outer shell.

He said his team would be happy to study the find together with the Swedish divers.

"We are ready to conduct a joint expedition," he said, adding that if the wreck is confirmed as the Catfish it would be important to also "immortalise the memory" of those who perished.

Stefan Hogeborn, a diver with the Ocean X Team that made the discovery, said the mini-sub was "completely intact" with "no visible damage to the hull" and the hatches were closed.

"It is unclear how old the submarine is and how long it has been laying at the sea floor, but the Cyrillic letters on the hull indicate that it is Russian," he said in a statement Monday.

In October, Sweden's navy launched a massive hunt for a foreign submarine, suspected to be Russian, in the Stockholm archipelago.

The military subsequently confirmed that "a mini-submarine" had violated its territorial waters, but was never able to establish the vessel's nationality.

Last year's hunt for the mystery vessel came at a time of particularly high tensions between Russia and the West over the conflict in Ukraine.