The search for a mystery "foreign vessel" off the coast of Stockholm is continuing after Sweden's military presented photographic evidence of the unidentified craft.

Around 200 troops, several stealth ships, minesweepers and helicopters have been scouring the sea around islands about 30 miles east of the country's capital

Triggered by a tip-off to armed forces about a "man-made object", military spokesman Dag Enander said the search was continuing "unabated".

"We continue the search during the night, the only difference is that it is bright now," he told Dagens Nyheter.

Rear Admiral Anders Grenstad said a grainy photo taken on Sunday morning by a "source" was the third such sighting.

"This is not ours, it's a foreign vessel," he said.

He 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.

"This is not a submarine hunt"

Mr Grenstad said the sightings followed a pattern built up over several years but rejected media speculation that the armed forces were "submarine hunting".

He stressed that the mobilisation - one of the biggest since the Cold War - was an intelligence operation.

"This is not a submarine hunt, using weapons to combat an opponent," he said.

"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."

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

And he dismissed a report from Swedish media suggesting that a Russian emergency transmission had been intercepted, indicating that a Russian submarine 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," he said.

"We have no information about an emergency signal or the use of an emergency channel.

"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."

During the 1980s and early '90s the then-neutral - and now non-aligned - Nordic country was regularly on alert following Russian submarine sightings.

In one notable case in 1981, a Soviet sub ran aground several miles from one of Sweden's largest naval base.