Twenty people were killed on board a Russian nuclear submarine while on sea trials, the navy said today.

The accident, which happened while the submarine was in the Pacific Ocean, was the deadliest to hit Russia's navy since the Kursk nuclear submarine exploded beneath the Barents Sea in 2000, killing all 118 sailors on board.

Prosecutors investigating the latest incident said they suspected the victims died after inhaling a toxic gas used as a fire suppressant when the vessel's fire extinguishing systems went off unexpectedly.

It was not clear why the portable breathing gear usually issued to Russian submarine crews did not save them. A navy spokesman said the nuclear reactor was not damaged and the vessel was now in port.

"Twenty people died," the Prosecutor-General's Office said in a statement. "Results of a preliminary investigation show that death occurred as a result of freon gas entering the lungs."

Twenty-one people were injured and taken to a military hospital in Vladivostok. Many of those on board were civilian workers from the shipyard that built the submarine.

Vera Sanzhonova said she had driven to Bolshoi Kamen, the naval base where the submarine was moored, to seek word on her husband, a civilian technician who was on board.

"I have not received any news. My husband is neither on the list of those injured nor among the dead," she told Reuters.

"I've been driving here all the way down from Vladivostok, shedding tears and swallowing pills."

Asked if there was a hope her husband was alive she said: "None at all, just despair."

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

The navy said 208 people -- or nearly three times more than its usual crew -- were on board the sumbarine. Seventeen of the dead were employees of Amur Ship-Building Enterprise and three were sailors, prosecutors said.

"It is possible that some of the people lingered (putting on their breathing apparatus) or they did not have the apparatus at all," Ruslan Pukhov, director of military think tank the Centre for Analysis of Strategies and Technology, told the Ekho Moskvy radio station.

The navy did not identify the submarine. Russian news agencies quoted naval sources as saying it was the Nerpa, classified by NATO as an Akula-class attack submarine.

Media reports said Russia had planned to lease the submarine to the Indian armed forces but there was no confirmation of this. In Delhi, a navy spokesman said it had no plans to purchase any submarines from Russia at the moment.