Russia has accused Ukraine of sending attack helicopters across the border to strike an oil storage facility in what would be the first raid on Russian soil since the outbreak of the war if confirmed.
Ukraine has not confirmed that it launched the attack, raising questions about whether Russian negligence may be to blame.
A Russian governor in the border region of Belgorod said that early on Friday two Ukrainian Mi-24 helicopters crossed the border at low altitude before firing rockets at an oil facility 25 miles from the border.
A video shared on Twitter shows a blaze near apartment blocks in Belgorod, some 40km (25 miles) from the border.
Some clips appear to show rockets hitting the oil depot.
However, Ukrainian aircraft have not struck targets in Russia previously. Governor Vyacheslav Gladkov's claim was not confirmed by Ukrainian officials.
Belgorod, a city of 370,000, lies just north of Ukraine's second city Kharkiv, which has been heavily shelled by Russian artillery and remains surrounded by Russian forces.
Governor Gladkov said in a Telegram message "there was a fire at the oil depot because of an air strike carried out by two Ukrainian army helicopters, which entered Russian territory at low altitude".
"Nobody was killed," he added.
An explosion also took place earlier on Thursday at the site of an arms depot in Belgorod, raising speculation that saboteurs were targeting the city, which has served as a major hub for Russian military units involved in the invasion of Ukraine.
The Russian news agency Tass reported that four servicemen were injured due to that explosion.
The explosions in the Russian rear came after Russia announced it was winding down its offensive toward Kyiv after its troops met heavy resistance in the month-long war. Western officials have warned that Russia may be repositioning its troops for a larger attack on Ukraine’s east.