Finnish military officials said on Tuesday that they had repelled an unidentified underwater object believed to be a submarine in the latest sighting of suspected Russian submarines in waters worryingly close to Nordic and Baltic nations.

The navy said it had noticed the underwater target near Helsinki on Monday and Tuesday morning, and fired some depth charges to warn it off, Reuters reported. Defense Minister Carl Haglund said the target 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," Haglund told local media, according to Reuters.

Last October, Sweden had spotted two unknown underwater vessels near its waters, leading to the country's biggest military mobilization since the Cold War. Stockholm said it had intercepted a Russian distress signal, but Moscow refuted allegations that it was behind the incursions. In response to the incident, the country’s military called for a $700 million budget increase to upgrade the navy’s sub-hunting capabilities.

Latvia has reported at least three sightings of Russian military vessels off its coast since November. And, earlier this year, Britain’s Royal Air Force also scrambled two jets to intercept two Russian bombers seen off Cornwall. In another incident, a commercial plane flying over Irish airspace was forced to divert its path to avoid Russian bombers.

Earlier this month, Finland’s defense minister published a statement, along with other regional officials, announcing cooperation with Baltic nations to prepare against potential Russian aggression.

"Russia's leaders have shown that they are prepared to make practical and effective use of military means in order to reach their political goals, even when this involves violating principles of international law," the officials wrote in Norwegian daily Aftenposten. "The Russian military is challenging us along our borders and there have been several border infringements in the Baltics."