Sweden releases sonar image confirming 'foreign submarine' Published duration 14 November 2014

image copyright Reuters image caption The sonar image allegedly shows sub-sea tracks left by a mini-submarine

Sweden has released a sonar image it says is proof that a foreign submarine entered its waters in October.

The grainy image shows underwater tracks allegedly left by the vessel.

Several sightings of an unidentified object near Stockholm sparked a huge search and Sweden's biggest military operation in years.

Naval vessels and planes searched waters near the capital but were unable to identify any intruding vessel or say which country was responsible.

Suspicion fell on Russia, but its defence ministry denied any of its ships were involved.

"The military can confirm that a small U-boat breached Sweden's territorial waters," the head of Sweden's armed forces, Gen Sverker Goransson, told a news conference.

image copyright EPA/HANDOUT image caption The Swedish military handed out an image taken by a passerby showing an object near Stockholm

image copyright EPA image caption More than 200 troops, stealth ships and helicopters were involved in the hunt

Prime Minister Stefan Lofven warned against any further incursions.

"We will defend Sweden's territorial integrity with all available means," he said.

Cold War echoes

Soviet submarine sightings caused security alerts in Sweden during the Cold War in the 1980s.

In one incident, a Russian sub loaded with nuclear torpedoes ran aground near a Swedish naval base in 1981.

The latest search was called off without finding or bringing any vessel to the surface.

Sweden reportedly intercepted a distress signal in Russian, but Russia suggested it could have been a Dutch vessel, something denied by the Netherlands.

The episode raised fears about Russia's intentions in nearby states, following Moscow's role in the Ukraine crisis.

Sweden is not part of Nato but has tightened ties with the alliance. It is also increasing military spending.

image caption The search focused on Ingaro Bay - about 30,000 islands make up the Stockholm archipelago