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The vessel is believed to have been involved in an accident during a naval exercise in the Arctic Circle, which caused it to sink.
Details of the accident - which happened on Sunday - have only just been revealed by the Russians.
Time is running out for the crew - some of whom are already feared dead - as oxygen supplies are running low.
Ten Russian ships are reported to be at the scene and rescuers are said to have lowered a diving bell to the stricken submarine.
Admiral Vladimir Kuroyedov, commander of Russia's navy, said: "Despite all efforts being made, the probability of a successful outcome is not very high."
After initially saying there was still radio contact with the submarine, the Russian Navy later admitted there was only "acoustic contact" or the sound of crewmen pounding on the hull.
The five-year-old Oscar II class nuclear craft, one of Russia's most advanced submarines, is lying powerless 500ft (152m) down on the floor of the Barents Sea.
It is still not clear what caused the sub to sink. It seems unlikely it collided with another submarine and Britain and the United States have denied any of their vessels were involved.
The Russian Navy has dismissed suggestions of a nuclear accident. It said the Kursk was not carrying any nuclear weapons and there was no evidence of any radiation leak.
Britain and the United States have offered to help with the rescue operation but there has so far been no request from Moscow.
Experts say temperature inside the sub will be a big problem. With the power turned off, it will get very cold inside and there will be little room to move about.
The Kursk, built in 1994, carries a minimum of 107 personnel. It is a nuclear strategic vessel that can carry up to 24 nuclear cruise missiles, used mainly in combat with ships.
Since its first nuclear submarine was launched 40 years ago, the Russian navy has had an appalling safety record. At least 507 submariners are said to have died in a series of disasters.
Spending on defence has been dramatically cut in recent years. Former Russian naval officers have told of poor maintenance, shortages of safety equipment and a slapdash approach to safeguarding reactors.
Reports from US spy submarines claimed two onboard torpedo explosions caused the Kursk to sink.
According to sonar tapes and other recordings analysed at the US National Maritime Intelligence Centre, a rocket-propelled torpedo misfired during loading or launch causing an explosion.
As the captain struggled to bring the sub to the surface, a second torpedo warhead exploded, tearing a hole in the bow and killing most of the crew.
An official Russian inquiry concluded no-one was to blame for the disaster, which it said was caused by a faulty torpedo. But relatives claimed the result was a whitewash.
All 118 people on board died. The Russians later confirmed a final "SOS, water" had been heard on the 14 August but after that there was no contact from the submarine.
In October 2001 a Dutch salvage team used a giant barge to pull the Kursk off the seabed and take it back to shore.
Russian President Vladimir Putin was severely criticised for his handling of the crisis. He was on holiday at the time of the disaster and did not return to Moscow.
He later promised the bodies of the sailors would be recovered and returned to their families whatever the cost. According to the Russian press, 23 million roubles was paid in compensation to the families who lost loved ones on the Kursk.
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