Newspaper of reference Dagens Nyheter reported late Thursday that three teenage instructors and children at a sailing camp in the suburb of Lidingo had spotted the mystery vessel on June 28.

For about 20 minutes, they observed what they believed to be a dark grey or black submarine near the surface and watched it sail away from where they were sailing dinghies.

The teens took photographs and a short video of the object, which Dagens Nyheter published on its website.

The Swedish military, which learned of the incident several days later by word of mouth, sent two officers on July 4 to question the teens, aged 17 and 18.

"We are confident about the measures we took when we received this information. But I can't go into which measures were taken nor which conclusions were drawn," Armed Forces spokesman Jesper Tengroth told AFP.

"The military does not share Dagens Nyheter's view that this was a foreign submarine," Tengroth later told the newspaper.

But he refused to say whether the military had identified the object, and if so, what it was.

Dagens Nyheter said neither the Swedish military nor civilian submarines were active in the area at the time of the sighting.

Swedish politicians expressed surprise that they had not been told of the possible incursion.

"We haven't been informed of this previously. They've been sitting on this (information) for several months, they should have sorted out what this is all about," Liberal lawmaker Allan Widman, a member of parliament's defence committee, told Dagens Nyheter.

"We are of course going to make sure we get this information as soon as possible. Observations like this should be taken seriously," conservative Moderates MP Beatrice Ask told news agency TT.

As a result, Tengroth said the military would inform lawmakers "relatively soon".

Alleged submarine sightings are not uncommon in Sweden.

-- Massive hunt in 2014 --

In October 2014, Sweden launched a massive hunt for a foreign submarine, suspected to be Russian, in the Stockholm archipelago over an eight-day period.

The military subsequently confirmed "a mini submarine" had violated its territorial waters, but was never able to establish the vessel's nationality.

After years of massive military cutbacks in the post Cold War period, Sweden has in recent years hiked defence spending, much of it focused on upgrading capacity to detect and intercept submarines.

A non-NATO country, Sweden has also stepped up its military capabilities and exercises with the alliance, including the current Trident Juncture 18 exercises, amid signs of more assertive Russian behaviour in the Baltic region.

That has included Russian planes occasionally skirting or violating the national air space of neighbouring countries.