The Swedish military on Sunday presented photographic evidence of a mysterious "foreign vessel" off the coast of Stockholm but rejected reports of being on the "hunt" for a damaged submarine.

Ever since the armed forces received a tip-off about a "man-made object" off the coast of Stockholm Friday, 200 men, several stealth ships, minesweepers and helicopters have been searching the sea around islands about 50 kilometres (30 miles) east of the Swedish capital.

"This is not ours, it's a foreign vessel," Rear Admiral Anders Grenstad told reporters, pointing to a grainy photo taken Sunday morning by a "source" -- the third such sighting since Friday.

"He saw something that was on the surface and after he took the picture it disappeared again."

Grenstad said it was not possible to determine the nationality of the vessel due to the poor quality of the photo -- which shows a far-off dark object jutting out of the sea surrounded by surf -- but said the sightings followed a pattern built up over several years.

He rejected media speculation that the armed forces were "submarine hunting" and stressed that the mobilisation -- one of the biggest, barring purely training exercises, since the Cold War -- was an intelligence operation.

"This is not a submarine hunt, using weapons to combat an opponent. It's about collecting intelligence to establish that there is foreign underwater activity," said Grenstad, adding that an area east of the Swedish capital appeared "to be of interest to a foreign power."

"Later there can be a situation where it becomes a submarine hunt. We're not there now."

- 'Not singling out Russia' -

He dismissed a report from the respected daily Svenska Dagbladet on Saturday suggesting that a Russian emergency transmission had been intercepted, indicating that a Russian U-boat was in trouble in the area.

"From the information we have, we cannot draw the same conclusion as the media that there is a damaged U-boat. We have no information about an emergency signal or the use of an emergency channel," he said.

"We've not singled out Russia, but said it is foreign underwater activity... It can be a U-boat, a mini U-boat or divers in a moped-like underwater vehicle."

Grenstad also confirmed that a Swedish naval excercise, involving a Nato submarine from the Netherlands, had been under way off the Stockholm coast and further out in the Baltic Sea at the time of the first sightings.

In recent months, Sweden has seen an uptick in Baltic Sea manoeuvres by the Russian air force, which has fuelled speculation that Russia's military is now making a show of force against a new, more aggressive, incursion from across the Baltic Sea.

In one of two airspace violations in September, two SU-24 fighter-bombers allegedly entered Swedish airspace in what Foreign Minister Carl Bildt at the time called "the most serious aerial incursion by the Russians" in almost a decade.

During the 1980s and early 90s the then-neutral -- and now non-aligned -- Nordic country was regularly on alert following Russian submarine sightings, including one notable case in 1981 when a Soviet U-boat, the U137, ran aground several miles from one of Sweden's largest naval base.