Russian emergency crews late Tuesday doused the fire on a nuclear submarine which had been undergoing repairs in dry dock.

Firefighters had been struggling to put out the flames for several hours before finally resorting to flooding the dock and submerging the submarine.

"At this moment, the fire on submarine Orel, which was under repair, has been completely put out," said Yevgeni Gladychev, spokesman for the Zvyozdochka shipyard.

The vessel -- classified as Oscar-II by NATO -- is a cruise missile type sub which carries nuclear material and has two reactors aboard.

"The nuclear fuel had been unloaded from Orel before it was put up on dry dock. The reactor is turned off," Zhitomirsky told RIA Novosti agency.

"There are no weapons on board. It is clear that if the submarine was armed it would not have been accepted for repairs," said the shipyard's spokesman Nikolai Blinov.

Despite attempts by authorities to downplay the incident, reports said Russia's navy chief Admiral Viktor Chirkov flew out to the scene along with a slew of top naval brass.

A statement by the press service of the Zvyozdochka shipyard said that the fire started at 2:00 pm (1100 GMT) in the stern, when welding works impacted rubber-based hull insulation.

"Employees and crew left the submarine in an organised fashion," it said. "Nobody has been hurt."

- Billowing smoke -

Televised footage from the dock had showed black smoke rising above the giant 155 metre-long (508 feet) submarine.

A representative of the Severodvinsk fire service however told AFP that "you can't smell the smoke in the city" and that municipal firefighters were not being called in to help the shipyard's own fire department.

The website of the Zvyozdochka (Star) shipyard said the Orel -- part of Russia's Northern fleet based in the region of Murmansk in the Barents Sea -- has been under repair since November 2013 and that the work would take two years.

Severodvinsk is located in Russia's northern Arkhangelsk region, on the White Sea.

In 2011, the rubberised coating on another nuclear sub called the Yekaterinburg caught fire while it was under repair in the northern port of Murmansk, injuring nine people who inhaled noxious fumes.