The submarine's nuclear reactor was not damaged and background radiation levels in the naval testing zone where the accident occurred were "normal," a naval spokesman said.

"During sea trials of a nuclear-powered submarine of the Pacific Fleet the firefighting system went off unsanctioned, killing over 20 people, including servicemen and workers," said Captain Igor Dygalo, the navy's spokesman.

The high-speed attack submarine was being tested after a construction process that began in 1991 and became bogged down after the Soviet collapse.

State media said the vessel had been due to be delivered to India's navy.

Officials said three naval officers and 17 civilians had died in the accident. Dygalo said the victims included servicemen and shipyard workers who had been participating in the trials.

Autopsies showed the victims died from inhaling freon gas released into part of the submarine when its fire extinguishing system activated for reasons that are unclear, news agencies quoted Vladimir Markin, spokesman for the federal investigative committee, as saying.

Although the crew were issued with portable breathing devices, "it's probable the submariners didn't notice the inflow of gas and when they felt it, it was already too late," RIA Novosti quoted an unidentified official at navy headquarters as saying.

The injured were evacuated from the stricken submarine aboard an accompanying ship and were taken to hospital to be treated for poisoning, Pacific Fleet hospital officials said. Their lives were not thought to be in danger.

A Russian expert meanwhile said that according to a Ria Novosti news agency report a lack of gas masks among too many untrained civilians may have elevated the death toll.

Gennady Illaryonov, formerly a high-ranking naval captain and a specialist in maritime technology, said that an over-reliance on automated procedures aboard the Nerpa potentially increased the scale of the tragedy.

"I cannot exclude that among those civilians who found themselves on board, not everyone had the (necessary safety) equipment and that those who did may not have known how to use it," Illaryonov was quoted as saying.

The toll of dead and injured made the accident the worst involving a Russian submarine since the 2000 Kursk disaster, when 118 crewmen died.

The submarine itself returned to the port of Bolshoi Kamen, site of a large refitting shipyard, where the bodies of the dead were offloaded, a spokesman for the shipyard said.

Television pictures showed the hulking vessel, more than a 100-metres (330-feet) in length and with a bulbous tail section housing its sonar array, heading for the port.

The accident occurred Saturday and Dygalo said President Dmitry Medvedev had been briefed by Defence Minister Anatoly Serdyukov and had ordered a "full and meticulous" investigation.

Dygalo did not identify the submarine involved.

However, a source in the Amur shipyard administration named the vessel as the K-152 Nerpa, a submarine of the Project 971 Shchuka-B type, or Akula-class by NATO classification, RIA Novosti news agency reported.

The Nerpa was to be leased to the Indian navy, with New Delhi reportedly paying two billion dollars for the lease of two Akula-class submarines, with the option of eventually buying them.

In October officials from the Amur shipyard reported the launch of sea trials for the 8,140-tonne Nerpa.

The Echo of Moscow radio station reported that the shipyard had been beset by problems including failure to pay workers since the arrest of a shareholder last December. It said this had led to a decline in standards.

Federal investigators opened a criminal probe into the accident, Interfax news agency reported.

Dygalo told AFP that the submarine itself was not damaged and there was no radiation leak.

A total of 208 people were aboard the submarine, of whom 81 were servicemen while the others were naval technicians and specialists.

Following the Kursk disaster in 2000, in which a vast nuclear submarine sank in the Barents Sea after an explosion on board, the Kremlin was harshly criticised for its sluggish and secretive response.

In addition to the Kursk disaster, Russia has seen a string of mishaps with its naval submarines.

Nine sailors died aboard a K-159 submarine that sank in the Barents Sea in August 2003 while being towed to port for decommissioning.

In 2005, a mini-submarine of the Pacific Fleet got snared underwater in a fishing net, requiring the help of a British rescue team that arrived many hours later as the vessel's oxygen supplies were dwindling.

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