Sweden said Friday it had evidence that a mini submarine entered its waters in October, triggering a week-long hunt with echoes of Cold War stand-offs fuelled by speculation of a Russian incursion.

"The Swedish defence forces can confirm that a mini submarine violated Swedish territory. This is a serious and unacceptable violation by a foreign power," the supreme commander of the Swedish armed forces, General Sverker Goeranson, told reporters.

He added that it was impossible to confirm the vessel's nationality.

The search, which stirred up memories of cold War cat-and-mouse games with suspected Soviet submarines along Sweden's long, rugged coastline, involved battleships, minesweepers and helicopters as well as more than 200 troops.

Sweden's Prime Minister Stefan Loefven issued a stark warning against any future incursions.

"Those who consider entering Swedish territory illegally should be aware of the enormous risks it will mean for those involved in the violation," he said.

"We will defend our territorial integrity with all available means. The armed forces has the necessary power to prevent a foreign vessel from getting away with it... and to do so with military force."

- Uptick in Russian activity -

The military operation threw into relief Sweden's exposed position along the Baltic Sea, an area of immense strategic and economic importance that saw intense naval activity in Soviet times, and which in recent weeks has seen a marked rise in close encounters with Russian military aircraft.

The October submarine hunt was triggered after sightings of a "man-made object" on October 17 -- later matched by hundreds of reports from members of the public who thought they saw "something" in waters near Stockholm -- and evoked memories of dozens of dramatic Cold War U-boat hunts in the 1980s.

In the most dramatic incident, a Soviet submarine U137 ran aground close to one of Sweden's largest naval bases in 1981 and was only allowed to leave after a humiliating wait.

Apart from that incident and several confirmed visual sightings, Sweden has never produced hard evidence of a Russian submarine in its waters.

Tomas Ries, an expert at the Swedish National Defence College said Friday's announcement was a major breakthrough for the Swedish military.

- First evidence -

"During 10 years of obsessive Cold War submarine hunts, they never came up with anything, nothing at all," he told news agency TT.

"It's very uplifting to know that we have succeeded."

However Sweden's armed forces commander refused to disclose details of the evidence or comment on how the navy detected a submarine but failed to identify its origin, citing national security interests.

"How we have done that with our sensors I won't go into because that's top secret and that would reveal the capability that we have in our systems," Goeranson said.

Although Sweden has consistently declined to point the finger at Russia, the submarine hunt added to fears in the region about a more assertive Kremlin since the start of the conflict in Ukraine and an uptick in Russian airforce activity in the Baltic.

During the search Russia denied it had a vessel in the area and asserted it was a Dutch submarine on a joint exercise with the Swedish navy prior to the October 17 sighting, a claim rejected by the Netherlands.