Rescuers mounted a desperate undersea mission on Tuesday to evacuate 116 sailors trapped in a stricken nuclear submarine on the sea floor before the oxygen ran out. Reuters reports said a first attempt to dock a rescue capsule to the sunken submarine had failed due to heavy storms and a second had been launched. Dwindling oxygen supplies on the vessel were pushing rescuers to speed up their efforts. The crew of the Kursk, one of Russia's most modern submarines, have been able to communicate with the outside world only by tapping on the hull since an accident at the weekend forced them to shut down the reactor and let the craft sink to the bottom. But a Navy spokesman said the tapping was growing fainter. "The signal is getting weaker. Of course, the oxygen is running low, people just need to lie or sit down," the spokesman at the base at Severomorsk told Reuters. It was not clear how the signals were being monitored. Navy commander Vladimir Kuroyedov told RTR television: "All we know is that there are still people alive, and they are signaling SOS. "What remains is our hope, which leaves us fewer and fewer chances every day. Our calculations show that by August 18 they will run out of oxygen," he said. Earlier he had given a bleak assessment of the crew's chances of survival in a report to President Vladimir Putin, currently on vacation in a Black Sea resort, saying the prognosis was "very grim". "I am not a pessimist, I am a realist," he told RTR. Officials have been careful not raise hopes too far. The undersea evacuation is a complex maneuver made far more difficult by the fierce weather and the stricken submarine's position. A rescue vessel will have to join the ship's escape hatch and bring the men up in groups of 15-20. Interfax quoted navy commander Vladimir Kuroyedov as saying the rescue capsule had a three-man crew, who could see through video cameras and portholes to guide the vessel. A Northern Fleet spokesman at the fleet's base in Severomorsk described some of the difficulties by telephone. "The ships are being blown off their anchors. It is difficult for the ships to hold their course," he told Reuters. "All the ships and the capsule should be directed at a certain angle to ensure less resistance to the current. "The storms have calmed down somewhat, although waves are still high. But the current is more important." The submarine is tilted on the sea bed, with its bow pointing down, and also listing sharply to one side. As a result, "these capsules and devices are sliding off it and cannot join with the vessel," he said. Kuroyedov said rescuers were driven by fear for the sailors' lives. "Our lack of knowledge about the fate of the crew has marked all our work," he said. RTR showed him pointing out damage to the bow of the craft on a diagram. On the starboard side a torpedo hatch was wide open and a command tower was slightly damaged. On the port side, some bits of the vessel were scattered on the sea floor. Officials have said the damage may have been caused by a collision or some kind of explosion on board, but have been reluctant to give firm theories at this stage. There were conflicting reports of exactly when the accident took place. Norway said it had received official Russian reports that it happened on Saturday, a day earlier than officials had earlier said. Russia's Prime-Tass news agency quoted an unnamed official in the U.S. administration as saying on Tuesday that two U.S. submarines close to the Kursk heard a blast late on Saturday or early on Sunday. Moscow has so far declined offers of help from the United States and Britain. But Kuroyedov said a group of naval officers would go to NATO headquarters in Brussels on Wednesday to see what help the alliance could offer. Washington has two Deep Submergence Rescue Vehicles which can conduct rescue operations in depths of up to 610 m (2,000 feet) and evacuate up to 24 crew members at a time. Britain has put a deep search and rescue submarine on standby.