Fire safety violations during routine maintenance works are seen as the most likely cause of the blaze at a floating repair dock.

No deaths or injuries among the crew and shipyard workers have been reported. However, nine people were hospitalized due to smoke inhalation, a law enforcement source in the Murmansk region said. Two have already been discharged from hospital.

President Dmitry Medvedev has ordered the Prosecutor General’s Office to thoroughly investigate the incident and to punish those guilty.

Investigators have questioned about 50 witnesses and seized documentation at the shipyard.

The submarine will be handed over to the Navy within hours, Shoigu said.

Deputy Prime Minister Dmitry Rogozin who oversees the defense industry said it will take several months to complete the repairs and bring the submarine back into service.

Officials said there was no radiation leak because the reactors had been shut down before repairs began.

Wooden scaffolding around the submarine caught fire, which spread to the submarine's outer hull, Northern Fleet spokesman Capt. 1st Rank Vadim Serga said earlier.

Delta-class submarines have an outer skin of anechoic rubber, designed to absorb sound from sonars to make the boat harder to detect, but which could burn in a dry environment. The boat has a double hull of thick steel, however, which would protect its interior from external fire.

"There is no possibility of fire burning through the hull and no threat to on-board equipment," Serga said.

Military investigators have opened a criminal case under the statute “destruction or damage of military property due to negligence,” a spokesman for the Investigation Committee said.

The K-84 Yekaterinburg nuclear submarine is one of seven Delta-IV class submarines in service, all deployed in the Northern Fleet. It carries 16 Sineva (NATO classification "Skiff") submarine-launched ballistic missiles.

The Delta-IV class submarines are the core of the naval component of the Russian nuclear triad at present and may remain in service for another 10 years.