The search in the Baltic Sea evokes memories in Sweden of the stranding in 1981 of the Soviet submarine U137 deep inside Swedish waters, which sparked intense suspicion about the scale and motives of such incursions.
"The Armed Forces have on Friday received information about alien underwater activity in the Stockholm archipelago," the military said in a statement on Friday (local time).
It said on its website that information about the latest suspicious activity came from a trustworthy source, without providing details.
The stranding of U137 during the Cold war marked the start of a period when the navy repeatedly attacked suspected foreign submarines with depth charges in Swedish territorial waters.
Captain Jonas Wikstrom told a news conference around 200 military staff are involved in the current intelligence operation.
"We gather intelligence with the sensors on the ships, on the helicopters and on land," he said. "We take this very seriously."
Wikstrom said there had been no armed intervention. He declined to comment on who might be responsible for the activity, and whether the report had been about a submarine.
Sweden on Sept. 19 said two Russian warplanes entered its airspace during the week, calling the intrusion a "serious violation" and sending a protest to Moscow's ambassador in the Nordic country.
Countries in the Baltic Sea region have become increasingly wary of Russia's military ambitions since Moscow annexed Ukraine's Crimea region in March.
