The Kremlin ordered an investigation yesterday into an accident that killed 20 people and injured 22 on board Russia's newest nuclear submarine.

Rescue ships escorted the attack submarine Nerpa to a military shipyard at Bolshoi Kamen after sea trials were halted in response to Russia's worst naval accident since the Kursk disaster eight years ago. Investigators blamed the incident on an accidental release of freon gas from the vessel's fire-extinguishing system.

The circumstances of the accident on Saturday remained unexplained last night, however. It was also unclear why portable breathing equipment normally issued to Russian submarine crews had failed to protect those on board from the gas.

The Nerpa was carrying 208 people, three times its normal crew, during the trials off Russia's Pacific coast. Seventeen of the dead were civilian workers employed at the Amur Shipbuilding Enterprise that had built the vessel. Officials were examining what triggered the fire-extinguishing system and whether operating procedures had been violated, said Vladimir Markin, a spokesman for Russia's Investigative Committee.

President Medvedev told the Defence Ministry to conduct a “full and meticulous” inquiry into the first such tragedy since he took office in May. His predecessor, Vladimir Putin, was strongly criticised over the Kremlin's slow reaction to the explosion that sank the Kursk submarine and killed all 118 crew in August 2000, months after he became President.

The injured were transferred from the Nerpa to an accompanying ship and taken to Vladivostok, where they were being treated in a military hospital for poisoning. Captain Igor Dygalo, the Navy's spokesman, said that the submarine's nuclear reactor had been unaffected and was working normally. There was no risk from radiation, he added.

He said that the accident occurred in the front two sections of the submarine, which began sea tests last month and was due to be commissioned by the Navy this year. Experts said that those who died would have been suffocated because freon works by displacing oxygen to extinguish fires.

An unidentified naval official was quoted by the Ria Novosti news agency as saying that there may have been a failure in the alarm system that warns the crew when the firefighting system is switched on. Those on board might not have realised that freon was being released until it was too late.

Construction of the Nerpa began in 1991 but was suspended for years because of the economic crisis after the collapse of the Soviet Union. It is an Akula II class vessel, regarded as Russia's quietest and most effective attack submarine.

The incident is deeply embarrassing for the Kremlin as President Medvedev and Prime Minister Putin seek to project Russian military power around the world. The nuclear-powered warship Peter the Great is leading a task force to Venezuela for joint war games in the Caribbean this month, the first time that Russia's Navy has operated near America since the end of the Cold War. The Admiral Kuznetsov led a carrier strike group of 11 vessels backed by 47 aircraft off the coast of France and Spain for naval exercises in January.

Tragedies at sea

— In August 2003 the Russian submarine K-159 sank while being towed to harbour in the Barents Sea to be stripped of its nuclear reactors. Nine crew members died In August 2000 the entire crew of 118 sailors on board the Kursk submarine died after an explosion

— Forty-two people died when a fire broke out in the submarine Komsomolets in April 1989

— In 1949 the US submarine Cochino was struck by violent storms off Norway. Another submarine, Tusk, managed to save all but one of the people on board, but six of its own crew died

Sources: National Geographic, agencies, Times archive

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