Sweden stepped up a military operation off the coast of Stockholm on Saturday following what it described as “foreign underwater activity”.
Since Friday evening, ships, helicopters and about 200 troops have been involved in the search of the Baltic Sea following a tip-of from what it said was a trustworthy source.
Navy leaders later said they had widened their presence around the Stockholm archipelago, a group of 30,000 islands and islets.
The operation has brought back vivid memories of the final years of the Cold War when Sweden repeatedly hunted alleged Soviet submarines along its coasts with depth charges.
Swedish defence analysts have speculated that a foreign submarine may have been in the area to replace old spy equipment.
The operation comes amid increasing tension with Russia among the Nordic and Baltic states over the Ukraine crisis.
Last week, Finland accused the Russian navy of interfering with a Finnish environmental research vessel in international waters.