Swedish authorities have asked all civilian vessels to leave waters south of Stockholm as a major operation continues to try to track down a mystery foreign vessel.
In scenes reminiscent of the Cold War, Swedish ships and helicopters scoured the area in a hunt for what some observers think could be a Russian submarine.
Russia denies it has a vessel in the area.
Russia’s official government newspaper said either Sweden’s echo location equipment was faulty or “as the old saying goes, the eyes of fear see danger everywhere”.
Swedish prime minister Stefan Loefven said the search was being extended southwards to the open sea about 70 kilometres south-east of Stockholm, and more military exercises were being carried out in the Baltic Sea.
As the search expanded, the military warned the public to keep a distance of at least 10 kilometres from the operation.
Since Sweden’s defence forces received a tip-off about a “man-made object” on Friday, more than 200 men, several ships, minesweepers and helicopters have combed the sea around islands close to the Swedish capital.
“There is no submarine hunt underway, there is an ongoing intelligence gathering operation,” Mr Loefven said.
For many Swedes the sight of more than 200 members of the country’s armed forces on the hunt for a suspected hostile vessel has been evoking a spooky sense of deja vu, because reported sightings of Russian submarines regularly sparked security alerts during the Cold War.
“It’s very, very weird. I mean, I was an analyst looking at Soviet military forces in the ’80s and followed these submarine incursions that were taking place in Swedish waters quite closely,” said Tomas Ries, assistant professor at the Swedish National Defence College.
Mr Reis said a possible ‘Loch Ness monster’ scenario was unlikely.
“I think one can assume that the Swedish defence forces have got enough other indicators that they’re not releasing to make them determined to make this very massive investment,” he said.
“To my mind, that indicates that they are very sure about this.
“They are professionals, they know what they’re doing. They’re not chasing ghosts.”