Suspected submarine off Finland
The Finnish navy has been chasing "an underwater object" which it has failed to identify, forcing it to drop depth charges to warn the intruder it had been detected, officials said.
Defence minister Carl Haglund described the intruder, first detected off the Helsinki coast yesterday afternoon, as "theoretically, a possible" submarine.
"We determined that there was something there under the water... we dropped a couple of warning depth charges which cause a lot of noise but don't pose any danger to the possible target," Mr Haglund said.
"At this stage we don't know if it was a vessel or something else. What we do know is that our sensors detected sounds that indicate activity."
The depth charges were dropped after the object was detected a second time during the night, the navy said.
"Underwater objects are very hard to identify," Olavi Jantunen, the navy's maritime operations chief, said. "We'll analyse the material ... It's a question of days, even weeks before we can determine what we observed."
Parliamentary defence committee spokesman Jussi Niinisto said the military occasionally detects such activity but described the use of depth charges as unusual. The previous time the navy dropped depth charges to warn a suspected intruder was in 2004.
The incident comes in the wake of a lengthy hunt for a foreign submarine in the waters of neighbouring Sweden in October.
The Swedish military did not provide details but said it had obtained evidence of the intrusion with sensors. Officials never blamed any country, though most Swedish defence analysts said Russia was a likely culprit.
The underwater searches in Finland and Sweden come during a period of increased military activity in the Baltic Sea region, with several reports of airspace violations by Russian military aircraft.
Nato says its aircraft conducted more than 400 intercepts to protect the airspace of European alliance members last year, an increase of 50% from 2013.