Sweden has proof that a small foreign submarine was operating illegally in its waters last month, its top military officer said on Friday after a mysterious episode that triggered the country's biggest military mobilization since the Cold War.
More than 200 troops, stealth ships and helicopters scoured Baltic waters off the capital Stockholm in October after reports of foreign "underwater activity", but without finding or bringing to the surface any submarine.
"The military can confirm that a small U-boat breached Sweden's territorial waters. We can exclude all alternative explanations," the head of Sweden's armed forces, General Sverker Goransson, told a news conference.
He said Sweden had not been able to identify which country was behind the intrusion.
The submarine's presence was picked up by military sensors, Goransson said. Supporting evidence, he said, included a picture showing a bubble pattern typical of a diving submarine and a sonar image of tracks on the sea floor.
Prime Minister Stefan Lofven said intrusion into Swedish territorial water was unacceptable and that Sweden would bolster its capabilities in detecting and identifying such activity.
"We will defend Sweden's territorial integrity with all available means," Lofven said.
The submarine hunt reflected tensions in a region where governments are increasingly worried about Russian assertiveness because of Moscow's role in the Ukraine crisis.
Sweden has already said it will increase spending on its military, including up to 70 new fighter jets and new submarines, as it looks to reverse decades of underspending on its armed forces.
The Nordic country has also drawn closer to NATO in the past few years although the current government has ruled out seeking membership of the U.S.-led alliance.