Sweden's military has for a fifth day dispatched forces to the Stockholm archipelago after reports of "foreign underwater activity" in the area.

Battleships, minesweepers, helicopters and more than 200 troops have scoured an area about 30 to 60 kilometres from the Swedish capital since Friday following reports of a "man-made object" in the water.

Supreme Commander General Sverker Goranson said there was "probable underwater activity" off the coast of Stockholm and he was ready to use "armed force" to bring the mystery vessel to the surface.

Sweden released a hazy photograph of what might be a mini-sub on Sunday.

"The most important value of the operation - regardless of whether we find something - is to send a very clear signal that Sweden and its armed forces are acting and are ready to act when we think this kind of activity is violating our borders," the general said.

"Our aim now is to force whatever it is up to the surface... with armed force, if necessary," he added.

Despite widespread speculation that the "activity" is a Russian mini submarine - amid unconfirmed reports of intercepted transmissions to the Russian enclave of Kaliningrad on the other side of the Baltic Sea - authorities in Sweden have not singled out Russia in their comments.

Russian denial

Russia has denied having any submarine in the area, and pointed the finger at the Netherlands, which laughed off the claim, saying its submarine had already docked in the Estonian capital Tallinn after taking part in exercises with the Swedish navy.

Goranson said it was it was "extremely difficult" to locate submarines.

"We never succeeded in the past - and no one else has either."

Still, he said the massive military operation - which focused Tuesday afternoon on the island of Ingaro, just 30km from Stockholm - would continue for as long as necessary.

During more than a decade of hunting Russian U-boats in the 1980s and early 90s, Sweden never succeeded in capturing one, except in 1981 when the U137 ran aground 10km from one of Sweden's largest naval bases, triggering a diplomatic stand-off with Russia.

Sweden has since downsized its military significantly and has scrapped some of the resources it used to hunt for Soviet submarines, including helicopters equipped with sonar and anti-submarine weapons.

Early Tuesday afternoon, at least five naval ships were stationed for more than two hours in an area east of Ingaro, the closest reported point to the Swedish mainland since the operation began. 

The armed forces have said the military search operation is taking place in "an area of interest for foreign powers" and that throughout the years, several observations of suspected foreign underwater activity have been made in the same area. 

Tensions have risen around the Baltic since the military operation began with Latvian Foreign Minister Edgars Rinkevics calling it a potential "game changer" for the security of the region. 

Source: Al Jazeera and agencies