Moscow - A sailor was charged with activating the locked-down fire safety system on a Russian nuclear submarine over a weekend accident that asphyxiated 20 people, investigators said on Thursday.

"The inquiry established that a sailor set off the anti-fire system on board the submarine without authorization and for no reason," news agency Itar-tass quoted Vladimir Markin, a spokesman for Russia's top investigative committee, as saying.

Twenty people suffocated and 21 were hospitalized after the fire-extinguishing system on board the new Akula-class submarine Nerpa released a freon gas coolant into the vessel Saturday as it underwent a test run in the Sea of Japan.

Russian authorities said there had been no fire on board the ship.

Markin said the sailor "has confessed to his error," and faces up to seven years in prison.

It was the Russian Navy's worst accident since all 118 sailors aboard the Kursk nuclear-powered submarine died when one of its torpedoes exploded, sinking the craft in the Barents Sea in 2000.

A wider inquiry has been launched into criminal negligence aboard the vessel amid new doubts over the safety of Russia ageing military hardware.

"Investigators are also ascertaining the degree of culpability of the officials, both civil and military ones, in the tragedy and people's death," Markin said.

Military experts said Thursday that the sailor may be taking the fall for the deaths of the three sailors and 17 civilian engineers from a shipbuilding firm refitting the Nerpa.

"Only senior commanding officers have access to the fire safety system. It is impossible to simply activate the system, which is protected from unauthorized activation by multiple levels of confirmation," an unnamed official told Ria-Novosti.

Russian state-television Vesti showed an officer aboard the ship saying the sailor had his confession likely beaten out of him. He said the culprit designated by investigators had years of seafaring experience.

It was also unclear why those onboard were not equipped with standard breathing kits, they added. Russian tabloids quoted survivors saying their masks simply didn't work.

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.

Despite the tragedy, the Russian submarine will resume sea trials, a spokesperson for shipyard told Ria-Novosti on Thursday.

"The Russian Navy has said it will still commission the submarine ... The same shipyard team will take part in future trials," Marina Radayeva said.

The Nerpa was due to be delivered in 2009 to the Indian, Russian news agencies had reported, but arms industry officials denied those reports Thursday.

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. (dpa)