Swedish authorities called off their weeklong search for a suspected submarine in the Stockholm archipelago Friday, saying the presumed intruder had probably escaped into the Baltic Sea.

Naval and amphibious forces were ordered back to base, 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 Adm. Anders Grenstad said.

Sweden's military launched its biggest anti-submarine operation since the twilight of the Soviet Union last Friday after receiving what it said were credible eyewitness reports of some type of underwater craft in the archipelago that extends from the capital, Stockholm, into the Baltic Sea.

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

Military officials haven't blamed any country for the suspected intrusion, though most Swedish defense analysts say Russia would be a likely culprit.

Russia has suggested that the suspected intruder might be a Dutch submarine that participated in an exercise with the Swedish navy last week. The Dutch navy said that submarine had arrived in Estonia last Friday, when the Swedish military received reports of suspicious activity.

"I don't want to comment on what Russia says," Grenstad said. "I have never singled out any nation. This is intelligence work to determine what is in our waters and what nationality it is."

Sweden built up an anti-submarine force after a Soviet sub with nuclear weapons ran aground off its southern shores in 1981 but started dismantling the force as part of deep cuts in defense spending after the Cold War ended. Anti-submarine helicopters were phased out in 2008 and replacements are not expected until 2018.

Apart from cutting defense spending, Sweden has shifted its focus from territorial defense to international peacekeeping operations and abolished conscription. In 2012 Sweden had 20,000 troops on active duty and 200,000 reserves, down from 50,000 active-duty personnel and almost 600,000 reserves in 1999, according to statistics from the Britain-based International Institute for Strategic Studies.