Sweden's military says it has sent naval vessels, aircraft and home guard forces to investigate reports of "foreign underwater activity" in the Stockholm archipelago.

The Armed Forces say they launched an intelligence operation Friday in the archipelago after receiving information "from a credible source."

Armed Forces spokesman Jesper Tengroth wouldn't say whether a submarine had been sighted or give any other details.

The announcement was reminiscent of the Cold War, when Sweden's armed forces routinely hunted for Soviet submarines in its waters.

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.