Sweden's armed forces said Wednesday they were following a Russian submarine that entered the Baltic Sea a day earlier, calling it a "routine operation" in collaboration with its allies.

"A Russian submarine entered the Baltic Sea yesterday via the Great Belt," a Danish strait, the military said in a statement.

"The armed forces' jet fighters and warships met up with the submarine in the Kattegatt (strait between Denmark and Sweden) and are now following it," it said.

The military said it was "a routine operation taking place in close collaboration with our allies", adding that it had a "good overview of our immediate vicinity".

Tensions over the Baltic Sea have heightened since Russia's full-scale invasion of Ukraine in 2022.

Swedish Prime Minister Ulf Kristersson said in January tha

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