Sweden has boosted its military presence in the Stockholm archipelago to search for "foreign underwater activity" in the county's largest mobilisation of troops and ships since the end of the Cold War.
The search in the Baltic Sea less than 31 miles (50 km) from Stockholm began on Friday and this operation comes amid increasing tension with Russia among the Nordic and Baltic states over the Ukraine crisis.
Finland last week accused the Russian navy of interfering with a Finnish environmental research vessel in international waters.
Swedish military spokesmen said the information about suspicious activity came from a trustworthy source and that more than 200 military personnel were involved in the search.
Swedish daily Svenska Dagbladet, citing unidentified sources with knowledge of the search, said the military operation began after a radio transmission in Russian on an emergency frequency was intercepted.