The oil plant is one of a few built in Russia in the past decades.
The Novoshakhtinsk oil refinery in Russia's Rostov region, bordering Ukraine, suspended operations on Wednesday after what may have been a drone strike, regional governor Vasily Golubev said on social media.
He said a fire had broken out in the refinery's vacuum unit, and that fragments of two drones had been found on the territory of the refinery.
Golubev added that the fire had been put out and no one was injured. An investigation was under way.
Social media footage showed a drone flying towards the refinery, located just 8 km (5 miles) from the border with Ukraine, before a large ball of flame and black smoke billowed up into the summer sky.
Reuters was unable to independently verify the reports or the videos on social media. The plant's spokespersons could not be reached for immediate comment.
The refinery has an annual capacity of up to 7.5 million tonnes, and started operations in 2009.
Russia is also investigating the cause of a large fire that erupted at an oil storage facility in the city of Bryansk, 154 km (96 miles) northeast of the border with Ukraine, in late April.
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