Swedish naval ships, helicopters and ground troops combed waters near Stockholm Monday after credible reports of foreign undersea activity and a suspected Russian intrusion sparked a submarine hunt not seen since the Cold War era.

The Russian Defense Ministry denied there were any emergency situations involving Russian warships Sunday and suggested a Dutch submarine might have triggered Sweden's alert, after carrying out exercises in the area.

The theory, however, was dismissed by the Dutch defense ministry Monday, French news agency AFP reported. 

Countries around the Baltic Sea remained on edge as the submarine hunt continued for a fourth day, seeking signs of a foreign submarine or smaller underwater craft that officials suspect entered Swedish waters illegally.

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.

The military said the intelligence operation in the Baltic Sea -- less than 30 miles from Stockholm -- involved a few hundred people.

The military showed one picture of what appeared to be a craft surrounded by white ripples, Reuters reported. The photograph was taken by a witness who said the vessel then submerged -- one of three sightings that the military said were credible reports.

A Swedish official, speaking on condition of anonymity Monday, told the BBC that air space around the search area had been closed off.

Sweden hasn't said what foreign country it suspects -- and Moscow denies involvement. Swedish Rear Adm. Anders Grenstad told reporters he wouldn't speculate on the photograph or sightings except to say the region is "of interest to a foreign power."

"It could be a submarine, or a smaller submarine," Grenstad told reporters. "It could be divers using some form of moped-like underwater vehicle and it could be divers that don't have any business on our territory."

"This does not belong to us. It is a foreign vessel and we have no indications that there would be any civilians involved in underwater activity,” Grenstad added.

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 over the weekend.

When asked why an underwater vessel would enter waters off Sweden, the Swedish official told the BBC it’s a measure of readiness. "It's always about testing the Swedish response to see if we are awake and able to detect it... it's a way of testing our skills in detecting and locating," the official said.

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 that monitors vessels in the Baltic Sea.

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

Russia has several submarines based in Kaliningrad, a Russian enclave bordered by Lithuania and Poland, which faces out to Sweden, and a much bigger force near Murmansk on the Kola Peninsula.

The Russian Defense Ministry responded, saying Sunday there were no emergencies 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 are no extraordinary, let alone emergency, situations involving Russian warships."

A Russian defense ministry official suggested Monday that the vessel could be the Dutch Bruinvis, which had taken part in exercises off the coast of Sweden.

"To remove tensions in the waters of the Baltic Sea and to save money of the Swedish taxpayers we would recommend (Sweden) to turn to the naval command of the Netherlands for an explanation," the unnamed official was quoted by Russian news agencies as saying.

The Dutch defense ministry quickly responded to the Russian claim, saying its submarine was no longer in the area when the unidentified vessel was reportedly sighted.

"We participated in an exercise with Sweden with several ships, but it ended last week Thursday," Dutch defense ministry spokeswoman Marnoes Visser told AFP.

"The Bruinvis then departed for Estonia, where it has been at anchor in the Tallinn harbor over the weekend."

Visser suggested the Bruinvis was currently making its way back to the Netherlands Monday."The Dutch submarine is not involved and we are further not involved in any search action or such," she added.

Swedish radio says the search area has expanded to include more islands out into the Baltic, further away from Stockholm.

Vessels and troops have been deployed from Moeja, north of Stockholm, to Ornoe, south of the capital, using sensors in an operation "continuing day and night," Swedish news website The Local says.

The incident comes amid rising tension with Russia among Nordic and Baltic states, over Moscow's involvement in the Ukraine crisis. Countries in the 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.

In just over a month, Finland has complained of Russian interference with one of its research vessels, an Estonian intelligence officer has been reported abducted by Russian forces, and Sweden said two Russian warplanes entered its air space, calling the intrusion a "serious violation."

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.

The Associated Press contributed to this report. 

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