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Ukrainian helicopters carried out a strike on a fuel storage depot in the western Russian town of Belgorod, the local governor said on Friday.
"There was a fire at the petrol depot because of an air strike carried out by two Ukrainian army helicopters, who entered Russian territory at a low altitude," Vyacheslav Gladkov wrote on his Telegram channel.
Ukraine's Defence Ministry declined to comment directly on Russian allegations that its forces attacked the depot.
"Ukraine is currently conducting a defensive operation against Russian aggression on the territory of Ukraine, and this does not mean that Ukraine is responsible for every catastrophe on Russia's territory... I will not confirm or deny these allegations," ministry spokesperson Oleksandr Motuzyanyk told a briefing.
If confirmed, it would be the first reported Ukrainian attack inside Russian territory since Moscow sent its forces into Ukraine in late February.
Video images of the purported attack posted online showed what looked like several missiles being fired from low altitude, followed by an explosion.
Russia's state-owned Ria Novosti news agency said eight of the 16 tanks at the depot were on fire after the alleged attack. Each holds 2,000 cubic metres of fuel, it said.
Russian oil company Rosneft, which owns the depot, said that no one was hurt in the fire, but gave no information on the cause.
Mr Gladkov said areas of the city near the depot were being evacuated.
Belgorod is about 80 kilometres north of the Ukrainian city of Kharkiv, which Russian forces have bombarded heavily in recent weeks.