Sweden is continuing a military operation off the coast of Stockholm looking for what is officially described as “foreign underwater activity”.

More than 200 troops along with helicopters and mine sweepers are scouring the waters after reports that a Russian submarine might have had mechanical problems while on a secret mission in the area, possibly to replace old spy equipment.

The Russian Defence Ministry said on Sunday that there were no emergency situations in the Baltic involving its vessels.

Swedish intelligence operators are understood to have intercepted an emergency radio conversation in Russian from a location in waters close to Stockholm and Kaliningrad, the home of Russia’s Baltic Sea fleet.

The operation has brought back vivid memories of the final years of the Cold War when Sweden repeatedly hunted alleged Soviet submarines along its coasts with depth charges.

One woman said: “We need to show that we are powerful when something that shouldn’t be in our waters is in our waters.”

When asked what she thought about the military activity, she replied: “I am happy that the military are doing their job, as we need them”.