HMS Koster
The Swedish navy's HMS Koster was involved in the hunt for a foreign submarine in October 2014Reuters

The Swedish military is investigating reports that a sunken mini-submarine found off the country's east coast may be Russian.

Images captured by the Ocean X Team and Ixplorer, who discovered the wrecked submarine last week, appear to show Cyrillic letters on the hull, indicating that it is Russian in origin.

It is not clear how old the vessel is or how long ago it sank.

The submarine is 20m long and about 3.5m wide and has "no visible damage to the hull", Ocean X Team's Stefan Hogeborn said.

He noted that the hatches of the vessel are closed and that it is possible that the crew did not manage to escape when the submarine went down.

"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 indicates that it is Russian," Hogeborn added.

Sweden's Expressen tabloid said the wreck was found 1.5 nautical miles (2.8km) off the east coast of central Sweden.

One expert told the paper that the vessel may be a Russian "Catfish" submarine that went down in the Baltic Sea during World War I in 1916.

Swedish military spokesman Anders Kallin said it was too early to speculate on the origin of the vessel.

"We choose not to comment on it before we have seen more material. We will continue the analysis together with the company in the coming days," he was quoted as saying by the Reuters agency.

The discovery comes amid growing concerns in the Baltic region over Russia's military ambition following Kremlin's intervention and annexation of Crimea last year.

In October 2014, the Swedish navy staged a frantic week-long hunt for a Russian submarine that was alleged to have entered its territorial waters, but no vessel was sighted.