Swedish Navy minesweeper HMS Koster patrols in the Stockholm Archipelago, on Oct. 19. The Swedish military's search for evidence of suspected undersea activity in its waters entered its third day on Sunday amid reports of a suspected Russian intrusion. (Marko Saavala/AP)

Sweden’s biggest submarine hunt since the dying days of the Soviet Union has put countries around the Baltic Sea on edge.

In a scene reminiscent of the Cold War, Swedish naval ships, helicopters and ground troops combed the Stockholm archipelago for a fourth day Monday for signs of a foreign submarine or smaller underwater craft that officials suspect entered Swedish waters illegally.

Although Sweden has not linked any country to the suspected intrusion, the incident sent a chill through the Baltic Sea region, where Russian forces have been accused of border violations on land, sea and air in recent months.

Swedish military officials say there have been three sightings of the elusive craft since Friday, just 25 miles west of Stockholm.

On Sunday, they released a photograph taken at a distance of what they said could be the mystery vessel — a dark speck surrounded by foaming water.

(The Washington Post)

Military spokesman Jesper Tengroth said that more than 200 personnel were involved in the operation, but he stressed that, unlike Sweden’s submarine hunts in the 1980s, the military wasn’t using depth charges or other anti-submarine weapons.

Speculating on whether the suspected underwater intruder was linked to a mother ship, Swedish media zeroed in on an oil tanker owned by the Russian company Novoship, which had been circling near Swedish waters. On Monday, Novoship President Yuri Tsvetkov said that he was “flattered” by the attention but that the ship was charted for transporting oil from Russia to the United States and was drifting on standby awaiting loading orders.

The daily Svenska Dagbladet has reported that Swedish intelligence picked up distress signals suggesting a Russian mini-submarine had run into trouble in Swedish waters.

Countering such claims, a Russian Defense Ministry official quoted by the Tass news agency suggested that the search was triggered by a Dutch submarine that participated in an exercise with the Swedish navy last week.

The Dutch navy, in turn, said that submarine left Sweden on Thursday and had been in Estonia since Friday.

In the final decade of the Cold War, Sweden launched unsuccessful submarine hunts after a Soviet sub carrying nuclear weapons was stranded off its southeastern coast in 1981.

The events in the past days have sparked alarm across the Baltic Sea in Estonia, Latvia and Lithuania — three former Soviet republics already spooked by Russia’s intervention in Ukraine.

Estonia stepped up surveillance of its territorial waters. Lithuanians were concerned about the safety of a floating natural gas import terminal being transported on the Baltic Sea to the Lithuanian port of Klaipeda.

Meanwhile, Russian media suggested that the Swedes were overreacting.

The official Rossiiskaya Gazeta newspaper questioned whether there was any submarine, noting that the Swedes hadn’t found anything.

The submarine scare comes after a string of border incidents involving Russian forces that Western analysts say signal Moscow’s growing regional assertiveness.

The Finnish Environment Institute said last week that Russian military ships had twice intercepted one of its research vessels in international waters.

On Sept. 5, an Estonian security service officer was detained on the Russian border — Estonia and Russia disagree on which side of it — and is still in custody in Moscow.

Sweden and Finland, which are not NATO members, have reported airspace violations by Russian military aircraft in the past two months.

— Associated Press