Sweden's military sent navy ships, aircraft and home guard forces to investigate reports of "foreign underwater activity" in the Stockholm archipelago on Friday, according to Reuters.
The Armed Forces said they launched an intelligence operation in the archipelago after receiving information "from a credible source," Reuters reported.
"The Armed Forces have on Friday received information about alien underwater activity in the Stockholm archipelago," the military said in a statement on Friday, according to Reuters.
Chief of operations Jonas Wikstrom told reporters "a few hundred" people were taking part in the search, but officials wouldn't speculate on what foreign power could be behind the suspected intrusion Friday, according to Reuters.
"We gather intelligence with the sensors on the ships, on the helicopters and on land," Wikstrom said, Reuters reported. "We take this very seriously."
The operation was similar to that of the Cold War, when Sweden's armed forces routinely hunted for Soviet submarines in its waters, Reuters reported. In 1981, a Soviet sub carrying nuclear weapons got stranded off Sweden's southeastern coast, causing an 11-day diplomatic standoff before Swedish authorities allowed the submarine to return home.
Last month, Sweden's Foreign Ministry summoned the Russian ambassador to protest a Swedish air space violation by two Russian military aircraft, according to Reuters.
