The governor of Russia's Lipetsk region identified the plant as one in the city of Lipetsk, some 400 km (250 miles) north of the Ukrainian border, that is responsible for about 18% of Russian steel output.

He said a fire apparently caused by a drone strike had been extinguished at the plant, operated by Russian steelmaker Novolipetsk (NLMK), and there had been no casualties.

The Ukrainian source told Reuters the attack, a joint operation by GUR military intelligence and the SBU security service, had caused a major fire and staff had been evacuated.

"Raw materials from this enterprise are used to manufacture Russian missiles, artillery, drones. Therefore, it is a legitimate goal for Ukraine," the source said, without specifying the location of the plant.

Video posted on social media purporting to be from Lipetsk showed an explosion, with a orange flames illuminating the nighttime sky.

NLMK said on social media the fire had broken out at 1:40 a.m. (2240 GMT on Friday). It company did not immediately respond to a request for comment.

The Lipetsk plant specialises in flat steel, making 80% of NLMK's steel products.

Ukrainian drone attacks on Russian military and industrial targets have become increasingly common in recent months, notably hitting oil facilities that Kyiv says are vital to the Russian war effort, but Ukrainian sources had not so far claimed responsibility for an attack on a steel plant.

Russian authorities said Ukrainian drones had been downed over the Lipetsk, Kursk and Tula regions overnight.

(Reporting by Polina Devitt in London and Tom Balmforth in Kyiv; Writing by Felix Light; Editing by William Mallard and Kevin Liffey)