Finland says its navy fired small underwater depth charges on April 28 as a warning against a suspected submarine in waters near Helsinki.
The grenade-size charges were fired amid growing military tensions with neighboring Russia, fueled by Russian border incursions and air force flights into Finnish airspace, since the conflict in eastern Ukraine began a year ago.
Finnish Defense Minister Carl Haglund did not say whether Russia was involved in April 28 incident in the Gulf of Finland.
But Haglund said: "It is always serious if our territorial waters have been violated."
Reports of a submarine spotted near Stockholm last year led to Sweden's biggest mobilization since the Cold War.
Regional tensions were reflected earlier in April after a joint statement by the Nordic countries -- Sweden, Norway, Finland, Denmark, and Iceland -- that directly cited the Russian "challenge" as grounds to increase defense cooperation.