Finish authorities said on Wednesday that they dropped depth charges off the Helsinki coast after an unknown object, possibly a foreign submarine, was detected underwater.
“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,” Finish Defense Minister Carl Haglund said.
The military said in a statement that they responded to the incursion of an object into their territorial waters Monday around noon time. After the object was again detected early Tuesday morning, depth charges were used to give a warning.
According to the Finish authorities it will take days to analyze data gathered by underwater sensors.
“Underwater objects are very hard to identify,” Olavi Jantunen, the navy’s maritime operations chief, said. “We’ll analyze the material … It’s a question of days, even weeks before we can determine what we observed.”
The previous time the navy dropped depth charges to warn a suspected intruder was in 2004.
The incident comes at a time of heightened tensions between Europe and Russia following the incursion of Russian troops into parts of Ukraine.
According to NATO, Russian military aircraft intercepted European airspace over 400 times last year.
Over the past decade Russia has increased spending on its nuclear submarine force by 150 percent.
According to the DC-based Nuclear Threat Initiative, Russia currently has a fleet of 10 Nuclear Power Ballistic Missile Submarines.
In November last year, Sweden released evidence of what it said was an incursion of a submarine in the Stockholm archipelago.
While Swedish authorities did not link the submarine to a specific country, there has been wide speculation that the submarine in question came from Russia. Russia has denied that any of its ships were involved.
The Associated Press contributed to this report.