By Stephen Eke BBC correspondent in Moscow

The Kursk disaster three years ago, was the worst in living memory

The naval authorities say the vessel had been attached to pontoons and was being towed to the shore. There, it was to be dismantled, starting with the removal of its two nuclear reactors.

But five kilometres (three miles) from the town of Polyarniy, a storm broke out. One of the pontoons became detached. As a result, the submarine became unstable.

It sank in water 170 metres (558 feet) deep.

The naval authorities appear to have decided on a course of openness with the media. From the moment the news broke, they have given detailed reports of what is known about the accident, and how the rescue efforts are progressing.

After recovering one crew member alive, and the bodies of two who perished, they were keen to play down the chances of finding anyone else.

With the temperature of the water about 10C they said, no-one would survive being immersed for more than 45 minutes.

The early morning news that a nuclear submarine had sunk in the Barents Sea immediately filled Russians with horror.

On hearing the words "submarine" and "sunk", memories of the Kursk disaster are brought back. That accident, three years ago, was the worst in Russia's recent naval history, shocking Russia and the world.

PR disaster

The Kursk was one of Russia's newest and most modern submarines. But it became an underwater tomb for 118 sailors after an explosion on-board sent it plunging to the bottom of the Barents Sea.

A small number of the crew had survived the initial explosion - only to die, hours later, in unimaginable conditions, suffocating in pitch darkness.

The Kursk disaster was a public relations disaster for the Russian navy and the Russian authorities.

President Vladimir Putin stayed on holiday and said nothing. Officials found it difficult to admit the true scale of the accident.

Instead, they cynically lied. Russia stubbornly refused foreign assistance even though its navy lacked modern search and rescue equipment.

This time, Mr Putin, on a trip to Italy, responded swiftly.

'Not seaworthy'

"Of course, all reasons for the tragedy will be established," he told reporters.

Submarine experts have already started saying it is likely the accident resulted from "incorrect towing procedures".

A former commander of the submarine, retired Admiral Eduard Baltin has said the submarine, a K-159, was in bad shape as early as 1983.

"It was already sinking then," he told Russian media. "We could just about maintain ourselves under water, but on the surface it kept losing its positive buoyancy."

Months after the Kursk disaster, an official inquiry found that volatile torpedo fuel lay at the root of the accident. The torpedoes were of a design rejected by Western nations decades ago.

Russia's navy is still viewed with pride by Russians. Like the other armed forces, it suffers from underinvestment, unreliable equipment and chronic discipline problems - particularly bullying.