Russian news agencies quoted the Zvezdochka shipyard as saying the fire started during wielding operations in an engine room of the Admiral Kuznetsov.

An official in the Murmansk regional government, minister of information policy Alexandra Kondaurova, told AFP that 10 people were hospitalised, one of whom was in serious condition.

Television footage showed emergency workers near the ship, which was spewing dark grey smoke from one side. The Northern Fleet said in a statement to Russian agencies that hospitalised servicemen suffered from smoke inhalation.

Interfax reported that the fire had spread over an area of about 600 square metres (6,500 square feet) and quoted a source describing the blaze as "very serious".

Russia's industry ministry told AFP that once the fire is out, officials would launch a probe and evaluate the extent of damage.

The Admiral Kuznetsov, which is not nuclear powered, became fully operational in 1995 and is the flagship of the Russian navy.

The repairs were expected to be completed by the end of 2020, with the carrier resuming active service 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.

- 'Ship of shame' -

The ship led a naval group off the Syrian coast, with fighters from the carrier hitting more than 1,200 targets during its mission.

It also stopped off the coast of Libya in January 2017 and took on board military strongman Khalifa Haftar, who Russia has reportedly backed in the country's long-running civil conflict.

Britain's then-defence minister Michael Fallon dubbed the Admiral Kuznetsov Russia's "ship of shame" for its role in the Syria conflict.

The ship suffered several mishaps during the mission, including the crash of two fighter jet into the sea while trying to land on the carrier.

The repairs and upgrades -- with a reported cost of up to 62 billion rubles ($1.0 billion/880 million euros) -- were to focus on the ship's power plant and onboard electronic systems.

The collapse last year of a crane on the ship's deck, which killed one worker and caused major damage, raised concerns the work would be delayed and the carrier would remain out of commission beyond the 2021 deadline.

Russia's navy has battled problems with state financing, ageing shipyards and delays in fulfilling orders for new ships.

In the last six years, three fires have been reported on submarines under repair.

The Admiral Kuznetsov was to be the first of a new fleet of Soviet aircraft carriers, but the Russian navy has been hit by a huge drop in funding since the 1991 collapse of the Soviet Union.

The hull of what was to be its sister ship was sold by Ukraine to China, completed, and christened as the Liaoning.

The USSR had five aircraft carriers at the height of its power.