ST PETERSBURG (Russia) - SOME of the 20 men killed on a Russian nuclear submarine this weekend may have died because they lacked adequate training in how to respond in a fire drill, a former submarine officer said on Monday.

Russian prosecutors said the victims on board the Nerpa nuclear submarine died from asphyxiation when the fire extinguishing system was set off unexpectedly, releasing toxic freon gas - used as a fire suppressant - into the submarine.

The accident, the worst to hit Russia's navy since an explosion on the Kursk submarine eight years ago killed all 118 sailors on board, raised questions about the capability of the Russian military.

Mr Alexander Nikitin, a former navy captain who worked as a chief engineer on Russian nuclear submarines, said civilian technicians on board the vessel to take part in sea trials may have not known how to react when the gas was released.

'When the system for releasing fire suppressant is switched on, an alarm sounds, but they may not have heard the alarm or understood it was an alarm to indicate fire suppressant is being released into the compartment,' Mr Nikitin said.

'Since it is quite a rare alarm, they could have assumed it was some kind of different alarm,' said Mr Nikitin, who now monitors nuclear safety in Russia for the Norwegian environmental group Bellona.

Mr Nikitin said everyone on board the vessel would have had access to breathing apparatus. 'Most likely, they did not use it according to the procedures or as they were supposed to.'

The governor of Russia's Primorye region on the Pacific coast, where the vessel docked after the accident happened, on Monday declared Nov 11 a day of mourning in the region.

A Russian navy spokesman declined to comment further on the accident, which happened on Saturday in the Pacific Ocean.

About two thirds of the 208 people on board the submarine were civilians checking the vessel before handing it over to the navy for active service. Civilians accounted for 17 of the 20 dead.

'In the chaos that you have on these kinds of sea trials with a handover team, lots of things can happen,' said Mr Nikitin.

'These are not military people who strictly follow instructions.'

'In situations like these, there is a danger of more serious incidents, there are always heightened risks.'

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. -- REUTERS