Sweden's armed forces chief warned Tuesday it could use force to bring to the surface a suspected Russian mini-submarine its navy has been hunting for days as two new sightings of the vessel were revealed.

The new "observations" of the supposed "foreign" submarine were made on Monday, rear admiral Anders Grenstad said, after the country's top military commander declared it was ready to use "armed force" to bring the mystery vessel to the surface.

Battleships, minesweepers, helicopters and more than 200 troops have scoured an area about 30 to 60 kilometres (20 to 40 miles) from the Swedish capital since Friday following reports of a "man-made object" in the water.

Grenstad said the new sightings were made by the public -- the fifth in as many days -- but did not say why the navy were treating them as credible.

Supreme Commander General Sverker Goeranson had earlier said there was "probable underwater activity" off the coast of Stockholm and the navy was ready to act against the intruder.

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 U-boat -- amid unconfirmed reports of intercepted transmissions to the Russian enclave of Kaliningrad on the other side of the Baltic Sea, and the presence of a near stationary Russian oil tanker off Swedish waters since the operation began -- authorities in Sweden have not singled out Russia in their comments.

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.

- 'Difficult to locate subs' -

"We have not found any vessel. We consider that the reports... confirm something is happening. There is probable underwater activity," Goeranson told reporters, adding that 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 Ingaroe, just 30 kilometres 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 several miles from one of Sweden's largest naval bases, triggering an embarrassing diplomatic stand-off for Russia.

Early Tuesday afternoon, at least five naval ships were stationed for more than two hours in an area east of Ingaroe, the closest reported point to the Swedish mainland since the operation began. The daily Dagens Nyheter reported that one of the ships had "made contact" with something, but General Goeranson denied the claim.

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.

Lithuanian Prime Minister Algirdas Butkevicius told Swedish newpaper Expressen Tuesday that the suspected Russian activity was a "warning to the Baltic countries and Scandinavia".

"What began in Ukraine is visible in other locations. EU countries must now stand united... Only by being united can we meet these challenges and threats," he added.