Firefighters work at the site of fire at an oil depot in Belgorod region, Russia, on Friday. AP

A fiery explosion rocked a Russian fuel depot near the border around dawn on Friday, and Moscow said Ukraine had attacked the facility, but Kyiv denied any involvement. There was no independent confirmation of details about the incident.

Russian Defense Ministry spokesman Igor Konashenkov said two Ukrainian Mi-24 helicopters had entered Russian airspace "at an extremely low altitude” and attacked the civilian oil storage facility on the outskirts of the city of Belgorod.

He said the facility was supplying petroleum "to civilian transport only. The oil base has nothing to do with Russian armed forces.

Peace talks between Ukrainian and Russian officials resumed via video, but Moscow warned that the helicopter attack on a fuel depot in the town of Belgorod would hamper negotiations.

Kyiv would neither confirm nor deny it was behind the attack, with Foreign Minister Dmytro Kuleba saying he did "not possess all the military information."

A still image taken from video footage shows a fuel depot on fire in the city of Belgorod. Reuters

After over a month of a military campaign that has reduced parts of Ukraine to rubble, Moscow said in peace talks earlier this week it would scale back attacks on the capital Kyiv and the city of Chernigiv.

But Ukraine's President Volodymyr Zelensky said Russia was consolidating and preparing "powerful strikes" in the country's east and south, joining a chorus of Western assessments that Moscow troops were regrouping. "This is part of their tactics," said Zelensky in a late-night address.

"We know that they are moving away from the areas where we are beating them to focus on others that are very important... where it can be difficult for us," he said.

"In Donbas and Mariupol, in the Kharkiv direction, the Russian army is accumulating the potential for attacks, powerful attacks," he said.

Fears grew that the theatre of war may yet widen, as Russia accused Ukraine of an air strike with helicopters hitting energy giant Rosneft's fuel storage facility in the western town of Belgorod, around 40 kilometres from the border with Ukraine.

'Errors in calculation?'

Oleksiy Arestovych, an aide to Ukraine's president, said Kyiv was concentrating on repelling the enemy. "For what's happening on Russia's territory, the responsibility lies with Russia, and it's up to them to deal with what's happened there," he said in a video on Twitter. But the consequence of Russia's accusation was swiftly made clear by the Kremlin.

An Ukrainian soldier embraces his wife Dasha, 22; (R) Dasha checks her phone after a search for possible remnants of Russian troops. AP

"Of course, this is not something that can be perceived as creating comfortable conditions for the continuation of negotiations," Kremlin spokesman Dmitry Peskov told reporters, referring to ongoing peace talks.

Meanwhile Russia is battling unprecedented Western sanctions that have led multinationals to quit the country en masse.

On the ground, Ukraine's troops were beginning to regain control including around capital Kyiv and in the southern region of Kherson -- the only significant city that Russia had managed to occupy.

Agencies