Four people were injured and one person was missing after the accident involving the Admiral Kuznetsov at the PD-50 floating dock, the largest in Russia and one of the largest in the world, local authorities and media said.

"We are assessing the extent of the damage. A crane fell onto the deck from a height of about 15 metres (45 feet)," the head of Russia's United Shipbuilding Corporation Alexei Rakhmanov told the TASS news agency.

"It is clear there is damage to the hull and the deck. Fortunately, it happened on those parts that are not vital" to the work of the ship.

Local media said the floating dock being used to repair the vessel had sunk.

The accident occurred at 3:30 am (midnight GMT) at the repair shipyard near Murmansk, the governor of Murmansk region Marina Kovtun said in a video she posted on social media.

"Unfortunately, one person is still not found," she said.

In a second video, Kovtun said one of the hospitalised workers was in a serious condition and that "doctors are fighting for his life".

"Four workers were removed from the water with signs of frostbite and taken to a medical facility," the regional branch of Russia's Investigative Committee said in a statement on its website.

It added that the accident could be the result of "violations in the operation" of the ship repair facilities.

Rakhmanov later said the incident was caused by a sudden power cut which meant the dock did not function normally.

"During the exiting (of the ship) from the dock, the ballasting system did not work properly. This was due, as our colleagues say, to a sudden power cut from the shore," he told TV channel Zvezda.

But in a statement on its website, Russian state power company Rosseti said "the networks supplying the shipyard worked in normal mode".

The Admiral Kuznetsov is undergoing its first major repairs since 1997. They are expected to be completed by the end of 2020 for the warship to rejoin the Russian Navy in 2021.

The vessel was deployed to the Mediterranean in 2016 and early 2017 to strike targets in Syria and help Moscow ally President Bashar al-Assad recapture rebel-held areas in the war-torn country.

International media noted that the ship was belching clouds of black smoke as it sailed towards Syria via the English Channel in October 2016.