A foreign submarine recently trespassed in Swedish territory , officials there said Friday , but they could n't say what country the vessel belonged to . The Swedish military began an intense sweep of the waters off the coast of Stockholm more than three weeks ago . On Friday , the Swedish Armed Forces announced that its investigation confirmed that a submarine was indeed in Swedish waters . When the search started , the Swedish newspaper Svenska Dagbladet reported that the military became suspicious after Swedish intelligence picked up an emergency radio call in Russian . Russia denied that it had any vessel in Swedish waters . Russia flexes muscles with long-range bomber flights near U.S. shores . While confirming that there was a territorial violation , the military said that `` the analysis can not determine the nationality of the intruder . '' Swedish Prime Minister Stefan Lofven gave a warning to whoever the intruder may have been . `` The government considers this to be extraordinary serious , '' he said in a news conference . `` Those who consider to enter Swedish territory shall understand what huge risks it will mean in the future . '' He vowed to strengthen Sweden 's ability to identify illegal activities in its waters . Opinion : Russia taking jabs at Sweden . Sweden used naval , ground and air military assets to investigate the report of the submarine , the armed forces said . One military observation determined , with `` no doubt '' that the foreign vessel was a submarine . In a statement , the military declined to elaborate on its finding , saying it did not want to disclose details about its capabilities . But the armed forces did say that observations from the public helped in their investigation .
Norway believes that a foreign submarine from an unknown county was in its waters . Sweden considers it a grave crime . The police had been investigating for more than three weeks .