The governor of Russia's Voronezh region said on Saturday that emergency services were trying to put out a burning fuel tank at an oil depot.
More than 100 firefighters and 30 units of equipment were working at the site, Alexander Gusev, the governor said on Telegram.
Earlier on Saturday, a Russian security source told Reuters that Wagner fighters had taken control of military facilities in the city of Voronezh, about 500 km (310 miles) south of Moscow. Reuters could not independently confirm that assertion.