Fire engulfed one of Russia's biggest nuclear submarines while it was being repaired Thursday, sending flames and smoke billowing into the sky, but officials said all weapons had been removed and no radiation leak was reported.

Emergency workers launched a massive operation to douse the flames after the blaze broke out on the 11,740-tonne Yekaterinburg while it was docked in the the northern Murmansk region near Russia's border with Norway.

The sub had all of its nuclear missiles and conventional rockets removed before entering the Roslyakovo dock, a defence ministry spokesman said, adding that its two reactors had been switched off well in advance and there was "no threat of a nuclear radiation leak".

"The power unit was switched off and is now safe," ministry spokesman Igor Konashenkov told Russian state television.

The fire broke out on wooden scaffolding surrounding the submarine and spread to its outer hull, Northern Fleet navy spokesman Vadim Serga told the Interfax news agency.

"There is no threat to the onboard equipment," he added.

Eleven fire brigades and a navy fire boat were involved in the salvage operation, but television footage showed huge clouds off smoke billowing from the shipyard even after the flames had been contained.

A special helicopter was also dousing the flames with tonnes of water from above, the Murmansk region's TV-21 channel reported.

"I would say the flames reached about 10 metres (over 30 feet)," one unnamed witness told the station.

The Delta IV class vessel was commissioned by the former Soviet Union in 1985 and can carry up to 16 inter-continental ballistic missiles, according to Russian press descriptions of the submarine.

Russia is believed to have six Delta IV submarines, which form the backbone of its sea-based nuclear defences.

The worst disaster to befall the Northern Fleet in recent years was the sinking of the Kursk nuclear submarine in August 2000, which killed all 118 seamen aboard when it caught fire and exploded in the Barents Sea.

In a separate incident involving the military Thursday, an Sukhoi 24 fighter jet crashed on landing in the southern region of Volgograd, although both pilots managed to eject and were unharmed, the defence ministry said.

"The plane exploded during landing," said a ministry spokesman quoted by Interfax. "The crew ejected following orders."

The accident occurred at the end of a routine training flight at the Marinovka aerodrome, 60 kilometres from the city of Volograd.

Accidents involving military aircraft are fairly frequent in Russia.