Sweden is using boats, troops, and helicopters in the waters near Stockholm to search for "foreign underwater activity."
The Swedish military said the search -- which Swedish media says is for an unidentified submarine -- began on October 17.
Military analysts in Sweden said it is a Russian submarine, although government officials refused to comment on the nationality of the vessel.
The Swedish daily "Svenska Dagbladet" said the military had intercepted a distress signal in Russian that was sent to a naval base at the Baltic seaport in Kaliningrad.
The operation comes amid increasing tension with Russia among the Scandinavian and Baltic states over the crisis in Ukraine.
Finland last week accused the Russian navy of interfering with a Finnish environmental research boat.
Sweden said last month that two Russian warplanes entered its airspace in a "serious violation."
Based on reporting by Reuters, dpa, and "Svenska Dagbladet"