Swedish naval and air forces are broadening their search for a suspected Russian submarine off the waters of Stockholm, after sightings of an unidentified vessel and “credible information” received by the Swedish government spurred its navy into action.

The BBC reports that the Swedish government believes “foreign underwater activity” off its shoreline seems increasingly likely as the search involving high-tech patrol boats and helicopters enters its fourth day.

“Over the last few days, observations have been made in three different locations, and we consider all those observations to be very credible,” said Sweden Rear Admiral Ander Grenstad. “This area is of interest to foreign powers.”

Russia denies any “irregular situation” near Stockholm, suggesting perhaps the sightings were of a Dutch submarine participating in joint exercises late last week. The Dutch military denied that, claiming their vessel had already arrived in Estonia.

Unconfirmed reports suggest the vessel is damaged, and may have sent out a distress signal in Russian on an emergency radio frequency.

Soviet submarine forces frequently violated Swedish territorial waters throughout the 1980s. And Russian attack aircraft violated Swedish airspace last month, prompting the Scandinavian nation to scramble fighter jets to see off the intruders.