Six months after a Russian sub lurking off the coast of Stockholm triggered Sweden’s biggest naval mobilization since the Cold War, officials in Finland said Tuesday that the country’s military had fired underwater depth charges at a suspicious vessel in waters near Helsinki.

Finnish military officials said the vessel appeared to be a submarine, but they were not certain and did not have clear evidence the vessel belonged to Russia. They also said it appeared to have left the area after the dispatching of the depth charges — roughly the size of hand-held grenades.

The development comes amid months of claims by Western and Eastern European military officials that Russian fighter jets and navel vessels have increased the frequency of their surprise incursions into the airspace and waters of other nations. Moscow, in turn, has complained about what it says are Western provocations along its border, including expanded military supplies to Ukraine and joint military exercises with countries all along the border with Russia in recent months.

Following Moscow’s annexation of Ukraine’s Crimean Peninsula last year, officials from several Eastern European nations first noted an uptick in the unannounced incursions.

In October, NATO officials claimed the Russian air force had flown an Ilyushin-20 spy plane into Estonian airspace, triggering a swift reaction from NATO fighter jets patrolling the area. The incident came after Sweden had scrambled a fleet of vessels to search for a suspected sub sighted about 30 miles off the coast of Stockholm in the Baltic Sea.

The ongoing war between Moscow-backed separatists and Ukraine’s military, meanwhile, has put nerves on edge in several nations along Russia’s western borders.

Finland shares an 833-mile border with Russia, and distrust toward Moscow has been a running theme in Finnish politics and society since the 1939-40 Winter War, when the Soviet Union seized sections of Finnish territory. The animosity has escalated over the past year of Russian air force sorties and military border exercises in the area.

Finland’s navy said it noticed an underwater target off Helsinki on Monday and again on Tuesday before deciding to fire the depth charges.

Defense Minister Carl Haglund said Finland has rarely used such warning charges, telling the Finnish news agency STT: “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.”

Finnish Commodore Olavi Jantunen told the Helsingin Sonamat newspaper that the underwater charges were “not intended to damage the target, the purpose is to let the target know that it has been noticed,” according to a report from the Reuters news agency.

Moscow’s relations with both Sweden and Finland took a turn for the worse earlier this month after Russian officials complained that the two countries were considering closer cooperation with NATO.

“Contrary to the past years, Northern European military cooperation is now positioning itself against Russia. This can undermine positive constructive cooperation,” Russia’s Foreign Ministry said in an April 12 statement.

Finish Prime Minister Alexander Stubb countered that “no other country, of course, has a veto over Finland’s decisions.”

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