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,” 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. The military occasionally detects such activity but described the use of depth charges as unusual.

The incident comes in the wake of a lengthy hunt for a foreign submarine in Swedish waters in October. This during a period of increased military activity in the Baltic Sea region, with several reports of airspace violations by Russian military aircraft.