Moscow on Friday accused Kyiv of carrying out its first air strike on Russian soil, further dashing hopes of any deescalation in President Vladimir Putin's war against Ukraine.

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.

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 particular, he warned, the situation in the country's south and east was "very difficult".

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

Washington echoed that assessment, with a senior US defense official saying Russia's focus on Donbas could herald a "longer, more prolonged conflict".

Fears grew that the theatre of war may yet grow, as Russia for the first time on Friday accused Ukraine of an air strike with helicopters hitting Rosneft's fuel storage facility in the western town of Belgorod, around 40 kilometers (25 miles) from the border with Ukraine.

"There was a fire at the petrol depot because of an air strike carried out by two Ukrainian army helicopters, which entered Russian territory at a low altitude," Belgorod region governor Vyacheslav Gladkov wrote on messaging app Telegram.

The consequence of the accusation was swiftly made clear by the Kremlin.

"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.

'Go back to my work'

Russia launched its offensive on February 24 on its neighbor, expecting to quickly take Kyiv and topple Zelensky's government.

But a ferocious Ukrainian fightback and logistics and tactical problems scuppered such plans. Meanwhile Russia has faced 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.

Russian troops "are continuing their partial retreat" from the north of Kyiv towards the Belarusian border, said Ukraine's defense ministry.

Condemning the invading forces for "pillaging" along the way, the ministry also said that in some regions, the Russians were forcing small businesses to use the ruble.

Around Kyiv, civilians were trickling out of devastated areas as Ukrainian troops retook control.

"The shops are closed, there's no delivery of supplies. The bridge is also blown up, we can't go for groceries through there," said Karina Tkachenko, holding her three-year-old daughter Karolina in a pink bobble hat in her arms.

"I hope all this will end soon, and I will go back to my work," she told AFP.

In the southern city of Mariupol, civilians were still desperately waiting for help after weeks under heavy shelling with little water, food and electricity.

'Not clear'

Russian forces have encircled and relentlessly bombarded Mariupol in their bid to capture the city in order to join up the breakaway regions in Lugansk and Donetsk under control of pro-Moscow forces in the east with the Crimea peninsula, which it annexed in 2014.

Yulia, who arrived to safety in Zaporizhzhia on Friday in a private vehicle in a group of 13 people including two daughters, told AFP it took them two weeks and five attempts to get out.

Their previous bids were either halted by shelling or blocked by Russian troops.

Some 42 buses carrying 3,000 evacuees were on Friday seeking to get out.

"We don't know when they will arrive, it depends on the checkpoints,” said a volunteer who is helping to coordinate arrivals in Zaporizhzhia.

Repeated operations to set up a safe corridor for civilians to escape weeks of relentless Russian bombardments of Mariupol have collapsed.

The international Red Cross, whose team of nine staff in three cars were on the way to the city to help coordinate a rescue operation, said it remained "hopeful ... but it's not yet clear that (the evacuation) will happen today."

Energy battle

With his economy crippled by unprecedented international sanctions, Putin has sought to leverage Russia's status as an energy power, and warned that EU members will need to set up ruble accounts from Friday to pay for his country's gas.

"If such payments are not made, we will consider this a breach of obligations on the part of our buyers" and existing contracts would be stopped, Putin said.

The EU has joined the United States in imposing sanctions, and European Parliament President Roberta Metsola is expected in Kyiv on Friday in a show of support.

But the bloc has not imposed an energy embargo, and Germany, which imported 55 percent of its gas supplies from Russia before the war, insisted it will pay in euros or dollars as stipulated in contracts.

Berlin and Paris were also "preparing" for Russian gas to simply stop flowing, France's economy minister said.

But the costs of the tensions over energy supplies were already seen in consumer prices, which surged by a never-before-seen 7.5 percent in the eurozone, data published Friday showed.