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.

Prime Minister Stefan Lofven warned against any further incursions.

“We will defend Sweden’s territorial integrity with all available means,” he said.