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MOSCOW (AFP): One of Russia's biggest nuclear submarines caught fire Thursday while undergoing repairs in dock in the northern Murmansk region but experienced no radiation leak, officials said.

The 11,740-tonne Yekaterinburg had all of its nuclear missiles and conventional rockets removed before entering the Roslyakovo dock near Russia's border with Norway, a defence ministry spokesman said.

The fire began during the repair work and quickly spread from the shipyard's wooden structures to the submarine's outer hull, Northern Fleet navy spokesman Vadim Serga told the Interfax news agency.

Emergency workers launched a massive salvage operation involving 11 fire brigades and a navy fire boat but television footage showed huge clouds of 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.

A defence ministry spokesman told Russian state television that the submarine's 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," defence ministry spokesman Igor Konashenkov said.

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 class submarines, which form the backbone of its sea-based nuclear defences.