But there were no casualties and authorities were investigating the cause of the fire at the Novocherkassk power plant in Rostov, the governor, Vasily Golubev, said on the Telegram messaging app.

Russia's defence ministry said its forces destroyed 11 drones launched overnight over Rostov by Ukraine, which borders the region, but did not say if there was any related damage.

The Baza Telegram news channel, which is close to Russian law enforcement, said the fire at the plant was caused by Ukraine's drones, however.

Reuters could not independently verify the report. Ukraine made no immediate comment on the attacks.

Novocherkassk is one of the largest thermal power plants in Russia's southwest, its owner, OGK-2, which is controlled by a subsidiary of Russia's energy giant Gazprom, says on its website.

(Reporting by Lidia Kelly in Warsaw; Editing by Clarence Fernandez)