A Swedish military search for evidence of suspected undersea activity in its waters has entered its third day amid reports of a suspected Russian intrusion.

The military has declined to comment on the operation except to say it is searching for evidence of "foreign underwater activity."

Daily Svenska Dagbladet reported the Swedes had picked up an emergency message suggesting a Russian mini-submarine had run into trouble in Swedish waters.

Anders Nordin from the Swedish Maritime Administration says a Russian-owned oil tanker, which had reportedly been circling near Swedish waters for days, turned Sunday and sailed to the north-east.

Media reports said the tanker's moves might be connected to the search.

The operation was reminiscent of the Cold War, when Sweden's armed forces routinely hunted for Soviet submarines in its waters.