Sweden beefed up its military presence in the Stockholm archipelago on Saturday to scour its waters for "foreign underwater activity", a mobilization of Swedish ships, troops and helicopters unseen since the Cold War.
The search in the Baltic Sea less than 30 miles (50 km) from Stockholm began on Friday and reawakened memories of the final years of the Cold War when Sweden repeatedly hunted suspected Soviet submarines along its coast with depth charges.
There is now increasing tension with Russia among the Nordic and Baltic states - most of them European Union members - over Moscow's involvement in the Ukraine crisis. Finland last week accused the Russian navy of interfering with a Finnish environmental research vessel in international waters.
The Swedish military has said information about suspicious activity came from a trustworthy source, without providing details, and that more than 200 military personnel were involved in the search.
The Swedish daily Svenska Dagbladet, citing unidentified sources with knowledge of the hunt, said it began after a radio transmission in Russian on an emergency frequency.
Further encrypted radio traffic from a point in the archipelago and the enclave of Kaliningrad, home to the Russian Baltic fleet's headquarters, was intercepted on Friday evening after the Swedish search started, the newspaper said.
The Russian Defence Ministry said on Sunday that there were no emergency situations in the Baltic involving its vessels.
"Russian Navy ships and submarines are fulfilling their duties in the world ocean waters in accordance with the plan," Interfax news agency quoted a ministry spokesman as saying. "There has been and there are no extraordinary, let alone emergency, situations involving Russian warships."
Countries in the Baltic Sea region have become increasingly wary of Russia's military ambitions since Moscow annexed Ukraine's Crimea region in March following the overthrow of Kiev's pro-Moscow president by protesters.
Last month, Sweden said two Russian warplanes entered its air space, calling the intrusion a "serious violation" and sending a protest to Moscow's ambassador in the Nordic country.
Ships, helicopters and troops from an amphibious unit as well as the home guard combed the search area in Stockholm's archipelago. The forces include HMS Visby, a corvette that has stealth technology and equipment for anti-submarine warfare.
The Swedish military said on Friday there had been no armed intervention and declined to comment on who might be responsible for the suspicious activity, or whether the report had been about a submarine.
"We still consider the information we received as very trustworthy," Captain Jonas Wikstrom, head of operations for the search, told reporters. "I, as head of operations, have therefore decided to increased the number of units in the area."
Should the search find proof of foreign military activity in Swedish coastal waters it will represent the first real test of Prime Minister Stefan Lofven's center-left minority government only weeks after coming to office.
In 1981, a Soviet submarine known under its Swedish designation U137 was stranded deep inside Swedish waters not far from a major naval base in the neutral country, sparking intense suspicion about the scale and motives of such incursions.