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STOCKHOLM — 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 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 Russian Defence Ministry said 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." 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.

A Swedish Navy fast-attack craft patrols in the the Stockholm Archipelago, Sweden, on Saturday.PONTUS LUNDAHL / AFP - Getty Images
— Reuters