Sweden could use force in its search for a suspected foreign submarine, a senior Swedish naval officer has told the BBC.
Rear Admiral Anders Grenstad said if a submarine were discovered, weapons could be used to make it surface.
But the military operation is focused on gathering intelligence, he added.
Russia has denied suggestions that one of its submarines got into trouble near Stockholm last week after distress signals were reportedly intercepted.
There have been several reported sightings of a mysterious vessel off the Swedish coast, prompting the search operation.
Rear Admiral Grenstad, who is deputy chief of joint operations in the search, said he had "no clue" which country owned it.
"Everybody is speculating - that's what you get when you're hunting submarines," he added.
Local media said Sweden had intercepted a distress signal in Russian.
Russia also has several submarines based in Kaliningrad, a Russian enclave bordered by Poland and Lithuania and facing out to Sweden, as well as a much bigger force near Murmansk on the Kola Peninsula.
'Something is in our waters'
Rear Admiral Grenstad said the Swedes were trying to establish that the mystery vessel was indeed a submarine.
"We hate the fact that we have something in our waters - or we believe something is in our waters," he said.
The Rear Admiral went on to say: "If we find the submarine with our own sensors the captain of the ship has the possibility to use weapons to get it to stop whatever it is doing."
He appealed to the public to help in identifying the vessel by keeping camera or phones handy if they were in the islands near Stockholm.
Prime Minister Stefan Lofven has announced his government will increase spending on defence in its budget on Thursday.
Sweden's search, now in its sixth day, has been focusing on Ingaro Bay off Stockholm.
Ships equipped to find submarines are among the vessels taking part in the operation.
Soviet submarine sightings during the Cold War caused security alerts in Sweden in the 1980s.
Russia's military intervention in Ukraine this year has fuelled suspicion about its intentions towards other neighbouring states, notably in the Baltic.