Moscow -  Twenty people died on a new Russian nuclear-powered submarine during a test run in the Sea of Japan when the vessel's fire safety system malfunctioned,the Russian navy said early Sunday.

The system released a lethal freon gas coolant which was found by autopsy in the victims' lungs, investigators said.

It was unclear what triggered the fire-extinguishing system and why those onboard were not equipped with standard breathing kits.

The military has launched a criminal investigation into the disaster, a spokesman for Russia's top investigative committee, Sergei Markin, was quoted by Russia media as saying.

The high-speed attack submarine's atomic reactor was undamaged and radiation levels aboard the vessel were normal, a Russian Navy spokesman said.

The submarine, bogged under construction since the early 1990s, was slated for delivery to the Indian navy, state media said.

It has put into port at Bolshoi Kamyen, a navy base and major shipyard, where the bodies of the dead were brought ashore in Russia's Far East near the Pacific coast city of Vladivostok, news agencies said.

It was the Russian Navy's worst accident since all 118 sailors aboard the Kursk nuclear-powered submarine died when a torpedo exploded, sinking the ship in the Barents Sea in 2000.

The Kremlin was heavily criticized for its secrecy and not responding effectively to save lives in that tragedy.

President Dmitry Medvedev ordered the Defence Ministry to carry out a full inquiry, the Kremlin press service said in a statement Sunday.

"While carrying out sea trials of one of the nuclear-powered submarine in the Pacific Fleet, the firefighting system went off unsanctioned, killing over 20 people." Captain Igor Dygalo, the naval spokesman said.

"Among the victims were ship-building workers and servicemen."

Markin added in televised comments 22 people were hospitalized after being evacuated by an auxiliary destroyer craft overnight, he said. Fourteen civilians and six sailors were killed.

There were 208 people on board during the sea trials, but only 81 were servicemen while the rest were specialists the Amur Shipbuilding Factory, news agencies reported.

The firefighting system is designed to discharge toxic freon gas, the most serious of three safety methods to kill flames on board.

There should have been more than 220 portable oxygen masks according to experts interviewed by radio station Ekho Moskvye.

Dygalo said the submarine was only due to join the Russia navy's fleet by the end of the year.

"With full responsibility, I assure that the underwater nuclear reactor is functioning normally and background radiation levels are normal," Dygalo said. "The submarine is not damaged."

He did not specify the name or class of the vessel.

But unnamed officials at the Amur shipyard cited by Ria-Novosti identified it as the Nerpa, known under the NATO code name as an Akula II class attack submarine. They said the submarine had submerged for the first time last week.

The Nerpa was due to be delivered in 2009 to the Indian navy under a billion-dollar contract with New Delhi for the ten-year lease of at least two such Akula-class submarines.

Russia, the world's second largest weapons exporter, has seen its navy hit by a string of disasters since the Kursk submarine debacle despite a huge increase in the military budget and effort to renovate its ageing Soviet fleet.

The news comes as a Russian naval squadron crossed the seas to the US-patrolled waters of the Caribbean in a show of force this month to hold joint exercises with Venezuela, one of its largest arms clients.

Nine Russian sailors died in 2003 when a submarine sunk in the Barents Sea as the vessels was being brought in to be retired.

In 2005, seven sailors were rescued by a British using hi-tech mini-submarines to ferry out the crew of a small Russian sub who had spent days caught metres deep in fishing nets.