The search for a foreign underwater craft in waters off Stockholm has brought back memories of Sweden's submarine hunts during the Cold War and exposed a key difference.

Back then Sweden actually had a robust anti-submarine force.

Sweden, which is not a NATO member, has downsized its military significantly since the Iron Curtain fell and has scrapped some of the resources it used to hunt for Soviet submarines, including helicopters equipped with sonar and anti-submarine weapons.

The military says the helicopters used in the current search don't have that equipment.

As the search continued for the fifth day on Tuesday, the deputy operations commander condemned the undersea activity in Swedish waters.

"It's extremely serious that we're searching for something or someone who has violated Sweden's territorial integrity," Rear Adm. Anders Grenstad told reporters in Stockholm.

He said the military had received reports of two more sightings in addition to three recorded on Sunday but did not want to speculate on the type of vessel or object in question.

Earlier Tuesday, Prime Minister Stefan Lofven said that Sweden's military "needs to improve its capacity." He cited Russia's increasing military activity in the region but added that "we do not regard that as an immediate threat to Sweden."