Sweden’s armed forces have deployed planes and ships in the waters near the country’s capital due to possible “foreign underwater activity” in the Stockholm archipelago, said the country’s military.

The counter-intelligence operation was carried on Friday night and will continue all Saturday, the country’s military said.

"This will continue until we consider that we are done," Jesper Tengroth, press officer for the Swedish army, told Swedish TT news agency, as cited by The Local.

He added that Sweden’s forces are now trying "to verify the information we received yesterday, which we assess comes from trustworthy sources, and see whether it has any substance or not."

Tengroth didn’t reveal if there were any observations during the operation on Friday night. He also didn’t provide any information on the nature of the threat or which country is responsible for the activity.

Defense Minister Peter Hultqvist also told TT that he was receiving regular updates of the operation in Stockholm’s Archipelago. He said that the maneuvers involving at least 200 troops were "necessary and well-founded."

"I have full trust in their judgment," he added.

“We have support for the analysis work we are now carrying out in the area in the form of ships, planes and territorial army personnel. The area is heavily trafficked, which is why it’s important for us to go public with information about what we’re doing,” said a statement from Jonas Wikström, commanding officer of the operation, on Sweden’s armed forces website.

He spoke only about certain underwater foreign activity, without mentioning any submarines.

According to Sweden’s Svenska Dagbladet newspaper, the place of the suspected foreign threat was detected in the southern part of the Kanholmsfjärden, a major bay in the Stockholm archipelago, only 50 km from the city center.

A local witness, Robert Erikson, who was fishing off the island of Skarpö in Stokholm Archipelago at around 16:00 local time (14:00 GMT) told TT that he saw the start of the military operation.

"The coast guard came in over the inlet with an airplane and flew round and round for a long time," he said, "There was a large military boat that stayed further south on Kanholmsfjärden."

The suspected incident comes a month after Swedish authorities accused Russian jets of violating the country’s airspace. According to Sweden’s armed forces, two Russian Su-24 fighter-bombers were detected flying over the country in September.