Moscow has accused two Ukrainian military helicopters of attacking a fuel storage depot in the Russian city of Belgorod.
It is the first accusation of a Ukrainian airstrike on Russian soil since Moscow invaded its neighbour in late February.
A huge fireball erupted at the storage plant in the Freida industrial district of Belgorod shortly before 6am local time.
Footage of the alleged attack appears to show several missiles being fired from low altitude resulting in a huge blast that sends flames and smoke into the air.
The videos have not been independently verified.
Firefighters have been seen trying to tackle the blaze which is around 25 miles from the Ukrainian border.
Vyacheslav Gladkov, the governor of Belgorod region has claimed the depot was hit by a low altitude Ukrainian helicopter strike with the resulting blaze injuring two people.
‘There are casualties. Two people. They’re employees of the oil depot. They’ve been given first aid and their lives are not in danger,’ he said.
‘We are starting to resettle the residents of Pochtovaya, Makarenko and Konstantin Zaslonov streets to a safer location.’
However, Russian oil firm Rosneft, which owns the depot, said in a separate statement that no one was hurt in the fire, though it gave no information on the cause.
Ukraine’s defence ministry has not responded or commented on the incident.
The explosion in Belgorod comes amid Western claims that Russia aims to stage ‘false flag’ attacks as a justification for continuing the war in Ukraine.
Two days earlier Russia claimed Ukrainian forces had dropped a missile on a munitions depot some 12 miles from the border, also in Belgorod region.
Six missiles hit the facility in Krasny Oktyabr with four soldiers wounded, according to Russian sources.
At the time, Gladkov said authorities were waiting for the Russian defence ministry to establish its cause.
If the attacks on Russian territory are confirmed as Ukraine, it would mark a significant escalation in the war, and a change of tactics from Kyiv.
Until now, hostilities have been limited mainly to Ukrainian territory.