Since last week, the Russian submarines -- based on the Kola Peninsula in Russia's far north -- have been dispatched for manoeuvres in the Atlantic Ocean, according to information from the Norwegian Intelligence Service, first reported by broadcaster NRK and later confirmed to AFP.

"There's a lot of activity in the North Atlantic at the moment and Norway together with other Nato states are monitoring with air and surface capabilities," Brynjar Stordal, a spokesman for the Norwegian armed forces, told AFP.

This is the largest operation of its kind conducted by Russia "since the end of the Cold War by the number of assets operating at the same time," he added.

According to sources cited by NRK, the Russian submarines, which all left their base in submerged mode, are seeking to reach as far into the Atlantic as possible -- ideally waters west of Greenland -- without being detected.

The operation is planned to last about two months with the suspected goal "to show that they are able to threaten the east coast of the United States."

"Of course they're showing their capabilities both in numbers and as to what they can deploy at the same time," Stordal said.

On Saturday, Russian news agencies reported two nuclear submarines, the Nizhny Novgorod and Pskov, were preparing for "deep dives to test certain equipment and weapons" in the neutral waters of the Norwegian Sea.

"During these dives, which will last a week, their crews will practice tactical procedures and different techniques for deep manoeuvres," according to a statement from the Russian North Fleet, quoted by the agencies.

The Norwegian Sea has depths of several thousand meters, unlike the Barents Sea outside the Russian port of Murmansk which only has an average depth of about 200 metres.

According to Russian media the two submarines are so called Kondor-class -- or Sierra-class -- multi-purpose subs, which are equipped to launch cruise missiles and designed to detect and destroy enemy air and sea targets.