Finland fired a number of warning shots on a suspected foreign submarine that entered its waters without permission on Tuesday near its capital Helsinki.
Charges the size of grenades were fired in the vicinity of the underwater target, which entered Finnish waters for the second time in a matter of hours.
The unidentified object was originally detected by the military’s underwater surveillance network on Monday midday, after which surface ships were deployed to search the area.
"During surveillance of [Finland's] territorial integrity, the navy detected a possible underwater object at midday on April 27, 2015, within Finland's territorial waters close to the border outside Helsinki," Finland’s Defence Ministry said in a statement.
"A warning was given with light depth charges at three in the morning," the statement added.
Speaking to Finnish news agency STT, Finnish Defence Minister Carl Haglund did not confirm the owner of the vessel, and said the submarine most likely left the area after the incident.
"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," Haglund said.
It is highly expected the vessel belongs to Russia, which shares a 1,340-kilometre border with Finland and has been increasingly asserting its military dominance in the region as tensions flare between Moscow and the West.
A similar incident occurred near Sweden last October, forcing the Swedish navy to mobilise off the coast of its capital Stockholm.
Norway has also been on alert to the Russian threat posed in the wake of increasing Russian military activity near its shores.
This week, Norway’s Defence Minister Ine Eriksen Søreide announced plans to purchase an anti-aircraft battery and German-made Leopard 2 tanks as the country seeks to increase its defence budget by around half a billion dollars to modernise its army.
Countries in the Baltics and Scandinavia have particularly been on standby for a possible Russian expansion into Europe after Russia annexed the Crimean peninsula from Ukraine last year.
Last month, Russia held a five-day military drill which included approximately 40,000 servicemen, 41 warships, 15 submarines and 110 aircraft in the Arctic - the same time Norway conducted its own "Joint Viking" operation involving 5,000 troops in Finnmark county, which borders Russia.
In December, Russian President Vladimir Putin notably said plans to expand Russia’s presence in the resource-rich Arctic region is among key priorities for the military.
Russian ambassador to Denmark, Mikhail Vanin, also recently warned Denmark not to pursue its interest in NATO's missile defence system, telling the Jyllands-Posten newspaper in March that such a move would make Danish warships a target for Russian nuclear missiles.
Such talk led Sweden, Norway, Finland, Denmark and Iceland to issue a joint statement earlier this month calling for increased cooperation in the field of defence between Nordic countries in the face of the Russian “challenge.”
In response, Russia said improving ties between Sweden, Finland and NATO were a cause for “special concern.”