By Niklas Pollard
STOCKHOLM (Reuters) - Sweden boosted its military presence in Stockholm's archipelago on Saturday to scour its waters for "foreign underwater activity" in a mobilization of ships, troops and helicopters unseen since the Cold War.
The search in the Baltic Sea less than 31 miles (50 km) from Stockholm began on Friday and brought back vivid memories of the final years of the Cold War when Sweden repeatedly hunted alleged submarines along its coasts with depth charges.
The operation comes amid increasing tension with Russia among the Nordic and Baltic states over 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 about 200 military personnel were involved in the search.
In 1981 the Soviet submarine, known under its Swedish designation U137, 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.
Ships, helicopters and troops from an amphibious unit as well as the home guard were combing the search area. It includes HMS Visby, a corvette that has stealth technology and equipment for anti-submarine warfare.
"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."
The military said on Friday there had been no armed intervention and declined to comment on who might be responsible for the activity, and whether the report had been about a submarine.
Should the search come up with proof of foreign military activity in Swedish coastal waters it will represent the first real test of Prime Minister Stefan Lofven's center minority government only weeks into office.
Last month Sweden said two Russian warplanes entered its airspace, calling the intrusion a "serious violation" and sending a protest to Moscow's ambassador in the Nordic country.
Countries in the Baltic Sea region have become increasingly wary of Russia's military ambitions since Moscow annexed Ukraine's Crimea region in March.
(Editing by Alistair Scrutton and Rosalind Russell)