Pictures appear to depict a drone attack by the Ukrainian military on a Russian oil storage, which is only 190 miles from Moscow.
The 4am attack ‘blew up an oil depot in the settlement of Stalnoi Kon’, said regional governor Andrey Klychkov.
Unverified images on social media showed what appeared to be a single rupture on the side of an oil storage tank, blackened by soot.
The logo of Russia’s state-controlled pipeline operator Transneft can be seen on the tank, which state television said was believed to be empty. The alleged drone attack left a crater some 12ft deep, according to one report.
Since the start of Russia’s offensive in Ukraine, several fuel or ammunition depots in southern Russia have been attacked by drones or helicopters, with authorities blaming Ukrainian forces.
The latest apparent attack comes a day after Russia fired almost 100 missiles at cities across Ukraine, causing three deaths and cutting the power supply to millions.
Ukrainian official Anton Geraschenko drew attention to the depot blast without claiming responsibility.
He posted an image of the site in flames – which it was not possible to verify – and a caption: ‘Smoking in the wrong place reached the Oryol region.’
Pictures later showed significant damage to the oil storage complex.
The site is around 100 miles from the border with Ukraine.
If it was Ukrainian, it would be a record reach for a Kyiv attack on Russia.
There are no casualties. All emergency services are working at the site,’ said Klychkov.
On the previous night, there were reports of a similar incident in the neighbouring Bryansk region.
Telegram channel SHOT said explosions caused by drone-dropped ammunition were heard at the Zhecha oil terminal, about 30 miles from the border with Ukraine.
It comes as US president Joe Biden told G7 and Nato partners that a missile blast in eastern Poland yesterday was caused by a Ukrainian air defence missile, a Nato source told Reuters on Wednesday.
The blast, which killed two people, raised global alarm that the Ukraine conflict could spill into neighbouring countries.
Ukraine blamed Russia. Russia denied its missiles struck Poland.
Biden told reporters in Indonesia that the missile was unlikely to have been fired from Russia.