Sweden has closed off airspace and is ordering civilian vessels out of part of the Stockholm archipelago as its hunt for a foreign submarine - presumed to be Russian - intensifies.
In the latest dramatic development of a chase that has revived memories of the cold war and further heightened tensions in the Baltic sea, Swedish media reported that the armed forces had ordered civilian vessels to keep at least 10km away from military ships.
The airspace above part of the vast archipelago was closed on Monday afternoon, according to newspaper Expressen, on the fourth day of a search that has involved hundreds of military personnel in boats, helicopters and aircraft.
Sweden's armed forces on Sunday released a picture (below) of what is believed to be the submarine and said there had been three sightings of it - two on Friday and one on Sunday - as it appeared to move south.
The operation has sparked flashbacks to hunts of Soviet submarines in the 1980s, most notably when one ran aground off the southern Swedish town of Karlskrona.
Concern is growing across the Baltic region about Russia's intentions after numerous incidents, from incursions into the airspace of Sweden and Finland - the only countries in the region aside from Russia not in Nato - to the abduction of an Estonian intelligence agent.
The Swedish military said: "Due to operational reasons, the Swedish armed forces are prevented from disclosing any details surrounding the operation. But the commanding officer has the mandate to act and follows all regulations."
The drama began on Friday when Sweden's military said a credible source had informed it of "foreign underwater activity" in the large archipelago off Stockholm. The armed forces have refused to comment on the nationality of the craft they are seeking, which could be a mini-submarine, underwater vessel or full-sized submarine.
But Svenska Dagbladet, a Swedish newspaper that last year revealed that Russia had simulated a bombing raid on Stockholm, said on Saturday that intelligence officials had picked up a distress call on a frequency used by Russians.
This led to speculation that a damaged Russian submarine was somewhere in Swedish waters, further fuelled by the sighting of a Russian oil tanker in international waters off Stockholm that could be used in a rescue, according to defence experts.
Russia's defence ministry on Sunday denied that any of its submarines were in an emergency or irregular situation anywhere in the world. Russia pointed to a Dutch submarine that was on an official visit to Sweden last week as a culprit, according to local news agencies. But the submarine appears to have been in Estonia over the weekend.
The Swedish armed forces later said it was "probable" that there was foreign submarine activity in the archipelago, the highest risk assessment except verification.
The hunt for the submarine provides an early test for Sweden's new centre-left government, which is more hostile to the idea of Nato membership than its centre-right predecessor.
Concern has been raised over the ability of the Swedish armed forces to defend the country for more than a few days, especially the strategically important island of Gotland in the Baltic.
The three Baltic countries, all members of Nato, worry that Gotland could be used in any conflict as a base from which to attack them.
The Swedish military operation is being followed around the region. Edgars Rinkevics, Latvia's foreign minister, wrote on Twitter at the weekend: "Closely following events in the Swedish territorial waters, may become a game changer of the security in the whole Baltic sea region."
Sweden must be hoping it has improved its equipment from 30 years ago. In 1995, the Swedish prime minister was forced into the embarrassing admission that the armed forces might have been chasing otters - not Soviet submarines.
At the time, Sweden had been complaining loudly about the presence of submarines from the former Soviet Union in its waters. Yet that was called into question after the navy received new hydrophonic instruments showing that mink could have given off similar sound patterns.
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