Russian firefighters on Friday put out flames on a nuclear submarine undergoing repairs near Norway after almost a day-long blaze that raised concerns about the security of Russia's ageing fleet.

The rubberised coating on the Delta IV class submarine Yekaterinburg caught fire Thursday at a dock in the Far Northern Murmansk region, the latest serious accident to have struck Russia's navy in the past few years.

"The fire has been liquidated. There is no burning," Emergency Situations Minister Sergei Shoigu told a meeting of officials.

The emergencies ministry has repeatedly insisted there were no signs of above normal radiation in the area and Shoigu said radiation monitoring would revert to normal.

A military source quoted by Russian news agencies said the Yekaterinburg would now be taken in for repairs that had initially been scheduled in 2013. "Clearly, it makes sense to do this now," said the source.

Television pictures from the closed military Arctic Circle port of Roslyakovo, just over 100 kilometres (60 miles) east of Norway, showed emergency workers firing jets of water on to the smoking hull of the sub.

The smoke plume had been visible for kilometres around during the polar night, local media reports said. The emergencies ministry said over 400 people were involved in the firefighting efforts.

In a sign of the gravity of the incident, President Dmitry Medvedev dispatched Russia's military chief of staff Nikolai Makarov to the scene, the Kremlin said.

Russia's Investigative Committee said it had opened a criminal enquiry into damage of military property by negligence.

But it remains unclear what originally sparked the fire, which first hit wooden structures next to the 11,740-tonne Yekaterinburg before spreading to the submarine itself.

The investigative committee said nine people -- seven sailors and two emergency workers -- were hospitalised after inhaling noxious fumes but described their condition as satisfactory.

There had been a succession of contradictory reports after the fire broke out, with several officials earlier quoted as saying the blaze had been put out even as firefighting continued.

It was also not entirely clear why several sailors had stayed on board while the fire was still burning, although an earlier defence ministry statement said it was to monitor temperatures inside the vessel.

But Shoigu insisted that Russia was telling the full truth about the accident.

"Disinformation needs to be stopped. We have no secrets and there never were any. We need to give reliable information and we put it all online," he said.

"We need to stop any insinuations. There is no need to spoil people's moods before the New Year holidays."

The Russian foreign ministry took the unusual step of issuing an official statement on the incident in which it confirmed the defence ministry's information and promised to issue prompt updates.

The Yekaterinburg was commissioned in 1985 and along with five other such vessels forms the backbone of Russia's sea-based nuclear defences.

It comes equipped with 16 inter-continental nuclear-capable ballistic missiles that can each carry four warheads and was last involved in a sea-based test launch in July.

The Northern Fleet has been hit by a series of small accidents and a deadly disaster in August 2000 that killed 118 seamen when the Kursk nuclear submarine caught fire and exploded while at sea.

Russian Deputy Prime Minister Dmitry Rogozin said that the repairs of the submarine would be carried out within a few months and the accident would not hurt its military capabilities, RIA Novosti reported.

However some analysts said the accident appeared sufficiently serious to potentially remove the vessel from the fleet for years.

"This is a very serious blow to Russia's nuclear deterrence capabilities," independent political analyst Pavel Felgenhauer told AFP.

Ironically, the accident coincided with the handover by Russia to India of the nuclear-powered attack submarine Nerpa on a 10-year lease following more than two years of delays, the ITAR-TASS news agency reported.

The Nerpa suffered Russia's most recent deadly submarine accident in November 2008 that killed 20 sailors when a fire extinguisher released a deadly chemical that was accidentally loaded into the system.