Sweden has confirmed that a small foreign submarine illegally entered its waters in October, but said it's still unclear which country was behind the incursion.
The head of Sweden's armed forces, General Sverker Goransson, told a news conference November 14 the incursion into Swedish territory was a "serious and unacceptable violation by a foreign power."
At the same news conference, Prime Minister Stefan Lofven warned, "those who consider entering Swedish territory illegally should be aware of the enormous risks it will mean for those involved in the violation."
Lofven added, "We will defend Sweden's territorial integrity with all available means."
Sweden launched a massive submarine hunt on October 17 after receiving reports of suspicious underwater activity on its territory.
The week-long search, which involved battleships, minesweepers, helicopters and more than 200 troops, had prompted speculation that the source of the suspicious activity was a damaged Russian submarine.
Moscow has denied the claims, blaming a Dutch submarine for triggering the hunt.
The Netherlands has rejected the accusation.