HELSINKI - The Swedish military said Sunday it had made three credible sightings of foreign undersea activity in its waters during the past few days amid reports of a suspected Russian intrusion in the area.

Rear Adm. Anders Grenstad said the armed forces had observed the activity in the Stockholm archipelago and nearby coastal area, but declined to give details of an operation reminiscent of the Cold War, when Sweden's armed forces routinely hunted for Soviet submarines in its waters.

The armed forces published a photograph taken on Sunday by a passerby showing a partially submerged object in the water from a distance, but it was unclear what kind of vessel was in question.

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Grenstad told reporters that he wouldn't speculate on the photograph or sightings except to say the region is "of interest to a foreign power."

The military said the intelligence operation, involving a few hundred people, began Friday. It was sparked by information "from a credible source."

Grenstad said it had no information about any emergency messages suggesting a Russian mini-submarine had run into trouble in Swedish waters and could be damaged, as reported by the Svenska Dagbladet daily.

The Defense Ministry in Moscow said its submarines and ships have been "fulfilling their tasks in the world's oceans," according to plan, Russian news agencies reported.

"There have been no emergencies or accidents with Russian military vessels," an unnamed spokesman at the ministry was quoted as saying.

Anders Nordin from the Swedish Maritime Administration said a Russian-owned oil tanker, Concord, which had reportedly been circling near Swedish waters for days, started sailing in a northeasterly direction toward Russia on Sunday morning. But it suddenly turned around and headed back in the direction of Sweden, according to Marine Traffic, a website which monitors vessels in the Baltic Sea.

Media reports said the movements of the Liberian-flagged tanker might be connected to the submarine search.

In 1981, a Soviet sub carrying nuclear weapons was stranded off Sweden's southeastern coast, causing an 11-day diplomatic standoff before Swedish authorities allowed the submarine to return home.

Swedish officials wouldn't speculate on what foreign power could be behind the suspected intrusion Friday. Last month, the Foreign Ministry summoned the Russian ambassador to protest a violation of Swedish airspace by two Russian military aircraft.