Sweden's military on Sunday confirmed a second sighting in October of what appeared to have been a submarine in waters near Stockholm city centre, not long after a search had been launched for a suspected Russian submarine off the coast.

In November the Swedish military released images of tracks on the sea bed and an apparent submarine periscope which it said proved that "a mini submarine violated Swedish territory" between October 17 and October 24 and that "at least one vessel" was involved.

However, despite widespread speculation that the submarine was Russian, the military never identified its nationality.

On Sunday, Swedish daily Dagens Nyheter revealed that a member of the public also photographed what appeared to be a submarine tower on October 31 about two miles (3 kilometres) from the city centre, close to busy ferry traffic lanes.

"We were informed of a sighting and considered it trustworthy -- we had a unit in the area and sent it to investigate," military spokesman Philip Simon told AFP.

"We received a photograph but do not plan to release it."

A Corvette, the HMS Malmo, carried out a search and concluded that it was a "possible submarine" but no other details of the previously unknown hunt have been made public.

"It looks like a black submarine tower. You can also see several other ships that normally use this shipping route," a military source who had seen the photo told the paper.

"You don't go this far in towards Stockholm for the fun of it. This information points to a serious intrusion towards central Stockholm."

The week-long search after the previous sighting involved battleships, minesweepers, helicopters and more than 200 troops scouring an area 30 to 60 kilometres (20 to 40 miles) from the capital.

It had stirred up memories of Cold War cat-and-mouse games with suspected Soviet submarines along Sweden's long, rugged coastline.

The submarine hunt and a series of alleged airspace violations by Russian jets over the last year have led to increasing debate about Sweden's military capability and to calls for the non-aligned country to consider closer NATO ties.

An opinion poll published Sunday by the state Civil Contingencies Agency Sunday found that 73 percent of Swedes were concerned about developments in Russia compared to 45 percent a year earlier.

For the first time the annual poll found more Swedes were in favour of NATO membership (48 percent) compared to those opposed (35 percent).