No single country on its own can handle such an operation

Deputy Prime Minister Ilya Klebanov

Moscow has been seeking international help in funding the operation, and Dutch and Norwegian firms are expected to take part.

Speaking last year Deputy Prime Minister Ilya Klebanov, who is in charge of the vessel's future, said: "No single country on its own can handle such an operation."

The submarine has already spent a winter on the seabed, but some experts warned last year that it was better to wait until this summer before trying to raise it.

The logistics

Weight: 25,000 tonnes est. Length: 155 metres Depth: 108 metres Cost of salvage: up to $100m

It normally takes at least three months for a submarine's nuclear reactors to cool down, and attempts to move the vessel too early could have cracked the hull, releasing radioactive matter, according to Nils Bohmer of the Bellona Foundation.

"It's vital to ascertain the condition of the reactors first," he told BBC News Online last August.

The options

The Rubin research centre in Saint Petersburg, which developed the Kursk, has been studying ways of salvaging the submarine.

• Refloat: The 155-metre vessel could be lifted with the help of cables attached to platforms or by giant air cushions, and then towed back to base, according to Admiral Vladimir Kuroyedov.

  Another radical option being considered is to strap balloons made of a special hi-tech fibre along the hull, and pump them with enough air to raise the estimated 25,000 tonne vessel to the surface.

  The relatively shallow depths - of just over 100 metres - at which the Kursk is lying make the option of refloating the submarine fairly feasible, according to Paul Beaver of Jane's Information Group.

• Chop up: If the Kursk is too damaged to stay intact, experts say the wreckage could be sawn into pieces to be hoisted individually. Or the reactor compartment alone could be lifted out - although this would again require special, very expensive, equipment.

• Move to shallow waters: Alternatively, it could be dragged to shallower waters. This may be a short-term option used to recover the bodies. However, the journey could still be dangerous.

Another idea that was considered was to simply seal off the reactors and leave the vessel on the seabed.

The Russians have developed a special biological gel, which was used when the Soviet submarine, the Komsomolets, sank in 1989. The gel is said to block all cracks, and has the advantage of being lighter than conventional materials. Russian specialists claim this material works effectively for 500 years. However, it is also very expensive.

Marine graves

Even so, some experts have said this would be the safest option. And some of the crewmen's families have said they would prefer their husbands and sons to remain untouched.

"I don't want to get a coffin that I'm not allowed to open, that could be filled with stones and rags," said Anna Troyan, who lost her husband. "I would rather just come down her and throw flowers into the water."

The Kursk is the sixth nuclear submarine to sink since the 1960s.

Two of the sunken submarines have been American, the other three Russian - buried at depths of up to 4,800 metres.

Most of them have been left on the seabed because of the huge expense of lifting them.