Submarine violated Swedish waters: PM

Sweden has confirmed that a small foreign submarine illegally entered its waters last month, though it's still unclear which country was behind the intrusion.

In unusually stark language for the non-aligned country, Prime Minister Stefan Lofven warned that such incursions presented "enormous risks" for those involved and Sweden would defend its borders "with all available means".

The Scandinavian country launched its biggest submarine hunt since the Cold War on October 17 after receiving reports of an underwater craft in the archipelago that extends from Stockholm into the Baltic Sea.

It called off the search after a week, saying the vessel had probably escaped into the Baltic.

Military officials never blamed any country, though most Swedish defence analysts say Russia would be a likely culprit.

"It's impossible ... to confirm any nationality. But we can confirm the fact that it has been there," Supreme Commander Sverker Goranson said at a media conference with Lofven and Defence Minister Peter Hultqvist.

Goranson said the military obtained evidence of the intrusion with its own sensors, but declined to give details.