The Russian defence ministry has denied reports that one of its submarines got into trouble in the waters off Sweden.
The Swedish military has been searching the sea since Friday, following what the military said was foreign underwater activity.
It has denied looking for a submarine, and said that it was conducting an intelligence operation.
However, a local newspaper said Sweden had intercepted a distress signal in Russian.
Swedish Rear Admiral Anders Grenstad said the military based its judgement on "not only on current observations, but also on many previous ones from this very area... this is an area of interest to foreign powers".
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.
It 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.
Last month, another Swedish newspaper, Expressen, reported an incursion by two Russian SU-24 fighter bombers into national airspace. Sweden scrambled jets to see them off, it said.
A defence ministry spokesman in Moscow told reporters that the Russian navy's submarines and surface ships were "performing tasks... according to plan".
"There has been no irregular situation, let alone emergency situation, involving Russian navy vessels," he said.
Sources told Svenska Dagbladet newspaper that Sweden had begun a search after a radio transmission in Russian was detected on an emergency frequency on Thursday evening.
Encrypted radio traffic from a point in the Stockholm archipelago and Kaliningrad was later also picked up.
A corvette equipped with anti-submarine technology, the Visby, joined other vessels, as well as helicopters and amphibious troops, in scanning the search area.