Swedish authorities have called off az weeklong search for a suspected underwater vessel in the Stockholm archipelago, saying the presumed intruder has probably escaped into the Baltic Sea.

Naval and amphibious forces were ordered back to base on Friday, while some ground forces remained in the search area, military officials said.

"We assess that the [vessel] that violated our waters has now left,'' Rear Admiral Anders Grenstad said.

Sweden's military launched its biggest naval operation since the Cold War last Friday after sightings of a "man-made object" - later matched by hundreds of reports from members of the public who thought they saw something suspicious in waters near Stockholm.

Grenstad said it was probably not a large submarine, but a smaller underwater craft that could navigate shallow waters in the archipelago. He did not rule out that there were more than one.

Military officials have not blamed any country for the suspected intrusion, though most Swedish defence analysts say Russia would be a likely culprit.

The archipelago of 30,000 islands, islets and rocks is hard to patrol. During more than a decade of hunting Russian U-boats in the 1980s and early 1990s, Sweden never succeeded in capturing one.,

The Russian U137 submarine did run aground 10km from one of Sweden's largest naval bases in 1981, triggering a diplomatic stand-off with Russia.

Sweden has since cut its military significantly and scrapped some of its submarine hunting equipment, including helicopters equipped with sonar and anti-submarine weapons.

Observers said the suspected intruder vessel or vessels - if they were Russian - would fit into a broader pattern of growing Russian activity in the Baltic.

In recent months, Sweden has recorded an increase in Baltic Sea manoeuvres by the Russian air force - including an incident in September when two SU24 fighter-bombers allegedly entered Swedish airspace.

Source: Agencies