Swedish Royal Navy vessels searched for an underwater intruder in the island-studded waters east of Stockholm on Monday after three sightings of a suspicious vessel and media reports of a Russian-language distress signal four days ago.
The response to a possible security breach, involving more than 200 naval personnel and dozens of ships and helicopters, was the widest by Sweden in the post-Cold War period, a reflection of the heightened tensions among Russia's Baltic Sea neighbors since Moscow seized Ukraine's Crimean peninsula in February.
"We have a different political security situation in the Baltic now, which means we react more quickly and more clearly state that we don't accept this," Rear Adm. Anders Grenstad, deputy chief of operations at the Swedish Armed Forces, told reporters in Stockholm.
The navy considered it "probable" that there was unauthorized "foreign activity underwater," Grenstad said, after three sightings of a vessel in the area since Friday.
He did not speculate about which foreign power might be responsible. But the Swedish newspaper Svenska Dagbladet, citing knowledgeable military sources, reported that the search for a foreign intruder was prompted by the interception Friday of a Russian-language distress signal sent via an emergency channel to a receiver in the Russian exclave of Kaliningrad.
Kaliningrad, a wedge of Baltic coastline between NATO members Poland and Lithuania, is a major base for Russia's Baltic Sea fleet and the only northern port for Moscow's navy that is ice-free year-round.
The reports have intensified perceptions in the Baltic region that Russia is taking a menacing stand against its Western neighbors.
Stockholm authorities protested to Moscow last month after two Russian fighter jets penetrated Swedish airspace. Finland, which shares an 800-mile border with Russia, last week complained that a Russian naval ship was interfering with a Finnish environmental research vessel in international waters. And Estonia, another NATO member, says Russian agents abducted an Interior Ministry security official near the border in early September. The man is reported to be jailed at the notorious Lefortovo prison in Moscow on espionage charges.
The Swedish navy on Sunday released a grainy photograph taken by a civilian showing a vessel that reportedly submerged moments after the image was captured.
An unsigned statement posted on Russia's Defense Ministry website over the weekend said Moscow was unaware of any problems encountered by its naval forces.
The Swedish newspaper Dagens Nyheter, citing intelligence expert Joakim von Braun, said that the object sought by the Swedish navy appeared to be an advanced Triton-NN mini-submarine.
Von Braun also noted that Russian special forces are equipped with underwater survival gear and that if the vessel became disabled they might have abandoned it to await rescue on one of the thousands of small islands in the Swedish archipelago.