The Ukrainian military said that its forces had hit a Russian oil depot that is located hundreds of kilometres from the warring countries' shared border and used by Russia's air force.
"Ukrainian defence forces hit Russian oil depot in Engels" in the Saratov region, the the Ukrainian general staff said in a statement on social media, adding that, "this oil depot provided fuel to the Engels-2 military airfield, where enemy strategic aviation is based".
The region hosts a major air base for strategic bomber planes that form part of Russia's nuclear forces.
Ukraine has attacked the base with drones before, but there was no word on whether it had been targeted on this occasion.
Russian news reports said the fire was at an oil facility.
Unverified videos and photos published on social media showed a large fire burning with orange flames, sending thick clouds of smoke into the night sky.
Regional governor Roman Busargin said the cities of Saratov and Engels, on opposite sides of the Volga river, had been subject to a "mass drone attack" and there was a fire at an industrial site, but that there were no known casualties.
"There are sufficient forces and resources to localise the fire," he posted on Telegram.
Russia's defence ministry said in a statement that 11 Ukrainian drones had been destroyed overnight over the Saratov region, and 21 over other parts of Russia and the Azov Sea. It did not mention any damage.
The Engels air base is located about 730km southeast of Moscow and hundreds of kilometres from the Ukrainian border.
In December 2022, three Russian air force personnel were killed when a drone was shot down there.
Independent news outlet Astra said the oil depot where the fire was burning provided fuel for the air base.
