In what is seen as a major embarrassment for the Kremlin, which has repeatedly promised to tighten safety checks after a number of marine accidents in recent years, an accident occurred on the Nerpa, an Akula II-class attack submarine, due to a gas leak that killed 20 people .

According to the experts, the fatalities aboard the submarine in the Sea of Japan, were the result of Freon gas spewing into the craft after a malfunction in its fire extinguishing system.

The Nerpa was carrying 208 persons, three times its normal crew, during the trials off Russia's Pacific coast. Seventeen of the dead were civilian workers employed at the Amur Shipbuilding Enterprise that had built the vessel.

Officials were examining what triggered the fire-extinguishing system and whether operating procedures had been violated.

Further more, it is being probed as to why had the portable breathing equipment normally issued to Russian submarine crews failed to protect those on board. The injured were transferred from the Nerpa to an accompanying ship and taken to Vladivostok, where they were being treated in a military hospital for poisoning.

An unidentified naval official was quoted saying that there may have been a failure in the alarm system that warns the crew when the firefighting system is switched on. Those on board might not have realized that freon was being released until it was too late.

Captain Igor Dygalo, the Navy's spokesman, said that the submarine's nuclear reactor had been unaffected and was working normally. There was no risk from radiation.

President Medvedev has ordered a "full and meticulous" inquiry into the tragedy. Sergei Darkin, governor of the Primorye region, ordered officials to give families of the dead and injured a 100,000 rouble (£2,360) as compensation payment.

The accident will affect the ambitions of Kremlin to project Russian military power around the world.