Looking for His glorious appearing! Titus 2:13

Hunting for submarines: A Swedish military vessel patrols the Stockholm archipelago. Photo: AFP

Helsinki: The Swedish military's search for evidence of undersea activity in its waters has entered its third day, amid reports of a suspected Russian intrusion.

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

The armed forces said it had launched an intelligence operation involving a few hundred people in the Stockholm archipelago after receiving information "from a credible source".

Military response: The Swedish navy corvette HMS Visby takes part in the hunt. Photo: AP

Officials declined to give more information, except to say they were investigating possible "foreign underwater activity" in the archipelago and other nearby coastal waters.

The Svenska Dagbladet daily reported Sweden had picked up an emergency message suggesting a Russian mini-submarine had run into trouble in Swedish waters and could be damaged.

But Russia's Defence Ministry said Russian submarines and ships have been "fulfilling their tasks in the world's oceans", according to plan, Russian news agencies reported.

"There have been no emergencies or accidents with Russian military vessels," an unnamed spokesman at the ministry was quoted as saying.

Anders Nordin, from the Swedish Maritime Administration, said a Russian-owned oil tanker, Concord, which had reportedly been circling near Swedish waters for days, started sailing in a north-easterly direction towards Russia on Sunday morning.

But it suddenly turned around and headed back in the direction of Sweden, according to Marine Traffic, a website that monitors vessels in the Baltic Sea.

Media reports said the movements of the Liberian-flagged tanker might be connected to the submarine search.

Swedish officials would not speculate on what foreign power could be behind the suspected intrusion on Friday.

Last month, the Foreign Ministry summoned the Russian ambassador to protest a violation of Swedish air space by two Russian military aircraft.