Nine people were injured when a huge fire engulfed a Russian nuclear submarine as it was being repaired, Interfax news agency reported Friday, but officials said all weapons had been removed and there was no radiation risk.

Firefighters launched a massive operation to douse the flames after the blaze broke out Thursday on the 11,740-tonne Yekaterinburg while it was docked in the northern Murmansk region near Russia's border with Norway, sending flames and smoke billowing into the sky.

The fire was contained at 1:40 am Friday Moscow time (2140 GMT Thursday) nine hours after it started and was expected to be eventually put out "in the coming hours," Russian Minister for Emergency Situations Sergei Shoigu said as quoted by Interfax.

Nine firemen were suffering from smoke inhalation, an emergency situations ministry source was quoted as saying.

The defence ministry had said earlier that the blaze in the Roslyakovo dockyard near one of Russia's main naval bases had been extinguished.

"In order to prevent a new fire breaking out, the submarine will be lowered into the water before returning to its original position," defence ministry spokesman Igor Konashenkov added.

The vessel had all its nuclear missiles and conventional rockets removed before entering the dock, Konashenkov had told state television earlier, 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," he told state television.

It is not yet known how the fire started, but a spokesman for the Russian military prosecutor, quoted by Ria Novosti, said a criminal probe had been launched into the "reckless destruction or damage of military assets".

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 also doused 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.

Thursday's fire occurred near the closed town of Severomorsk, one of the Russian navy's main bases.

In a separate incident involving the military Thursday, a 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 (40 miles) from the city of Volgograd.

Accidents involving military aircraft are fairly frequent in Russia.