Sweden's military has withdrawn some of the ships taking part in a submarine hunt in the Stockholm archipelago.

Military spokesman Erik Lagerstedt said some of the naval assets, including the corvette HMS Visby, returned to a naval base Wednesday for "maintenance" as the search entered "a partially new phase."

He rejected the move was a de-escalation, saying the operation continued with air, ground and some naval assets.

The Swedish military launched its biggest anti-submarine operation since the twilight of the Soviet Union on Friday after receiving credible reports of foreign underwater activity in the archipelago that extends from the capital, Stockholm, into the Baltic Sea.

Military officials haven't blamed any country for the suspected intrusion, though most Swedish defense analysts say Russia would be a likely culprit.