The Finnish navy on Tuesday fired three warning depth charges at suspected foreign submarines floating underwater off the coast of Helsinki.
The charges, which are about the size of a grenade, were fired after Finland noticed an unidentified object underwater on Monday, and again on Tuesday morning.
"Our sensors detected something, an uncertain underwater object, at lunchtime on Monday and then again overnight," Captain Olavi Jantunen told Helsingin Sanomat newspaper, reported the Guardian. "Three charges were dropped, but they were warning charges, with a light burst, meant to warn rather than do any harm to the object."
Jantunen said it's impossible to determine what the object was until a complete analysis has been performed on the sensor data, which could take as long as three weeks, according to the Guardian.
The defense ministry issued the following statement: "As part of the tasks to protect territorial integrity, the maritime surveillance system alerted the Finnish Navy of a possible underwater target around midday on 27 April 2015. The possible target was located within territorial waters, close to the limit of territorial waters off Helsinki. A search was then conducted by surface vessels."
Finland's defense minister, Carl Haglund, told Finnish media, "It's always serious if our territorial waters have been violated."
He said he was reminded of an incident that occurred last October, when Sweden mounted its largest mobilization since the Cold War in search for a submarine spotted floating in its waters near Stockholm, which many thought to be of Russian origin.
Tensions have been strained in the Baltic region as of late due to an unusually active Russian military presence in the air and sea space of neighboring countries. Finland shares an 833-mile border with Russia, and the high number of NATO interceptions of Russian planes has left the country seeking to build a regional alliance.
Russian activity prompted Finland to announce that its military would begin cooperating more closely with its Nordic neighbors, including Norway and Denmark. The Nordic countries released a joint statement earlier in April, referring to Russia's presence as a "challenge" worthy of increased defense cooperation, reported the Telegraph.
Haglund did not comment on whether Russia was involved. Whatever the object was, he said, it has likely left the area by now.