Four people were injured and one person is missing after the accident involving the Admiral Kuznetsov at the PD-50 floating dock, the largest in Russia 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 meters (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 holding the vessel had sunk.

The accident occurred at 3:30am 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.

"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 ship repair facilities.

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 last year to strike targets in Syria and help Moscow ally President Bashar al-Assad recapture rebel-held areas in the war-torn country.