Finland's navy has dropped depth charges in waters near Helsinki as a warning to a suspected submarine.
An unidentified object was spotted on Monday within Finnish territorial waters. It was detected again early on Tuesday, the navy said.
The incident comes amid growing concern in the region over Russia's military exercises.
In October, Sweden launched a hunt for a foreign submarine suspected to have entered waters near Stockholm.
Navy operations chief Commodore Olavi Jantunen told Helsingin Sanomat newspaper that the depth charges, dropped at 03:00 on Tuesday (midnight GMT), were meant only as a warning.
"The bombs are not intended to damage the target, the purpose is to let the target know that it has been noticed."
Defence Minister Carl Haglund said the object seen in the Baltic Sea this week could have been a submarine.
"We strongly suspect that there has been underwater activity that does not belong there. Of course it is always serious if our territorial waters have been violated," he told Finnish news agency STT.
Commodore Jantunen was more cautious, telling Finnish public broadcaster YLE only that the sightings involved a "possible underwater object".
Finland has become increasingly worried about the military exercises of neighbouring Russia. The two countries share a 1,300km (800 mile) border.
But the defence minister did not say whether he thought Russia was involved.
Finnish Border Guard ship Turva patrolled the waters off Helsinki on Tuesday
Finnish media reported that although the target was believed to have left the area, the investigation into the incident would continue.
Finland had a close eye on its waters and it had gathered useful information about the target for further investigation, Commodore Jantunen said.
Finland is not part of Nato but it has strengthened its ties with the Western military alliance since the Ukraine crisis.
Earlier in April, Finland also agreed to increase defence co-operation with other Nordic countries in response to Russia's activity in Ukraine.
Last October, Sweden launched a week-long search for a suspected submarine in its waters.
Naval vessels and planes scoured the Stockholm archipelago, amid suspicions that a Russian submarine was in trouble there.
Russia's defence ministry denied any involvement.