KYIV, Ukraine:Ukraine refused Friday to deny it had carried out an airstrike on Russian soil, as hopes of de-escalation faded in President Vladimir Putin's war and evacuation operations for besieged Mariupol failed.
Peace talks between Ukrainian and Russian officials resumed via video, but the Kremlin warned the helicopter attack on a fuel depot in the town of Belgorod would hamper negotiations.
Kyiv would not be drawn on whether it was behind the attack, with Foreign Minister Dmytro Kuleba saying he did "not possess all the military information".
With the prospect of war expanding across Ukraine's borders, little progress was reported in one of the country's most pressing humanitarian disasters, in the shattered southern city of Mariupol.
The international Red Cross said a team heading to the city was forced to turn back Friday after "arrangements and conditions made it impossible to proceed".
It was the latest among several collapsed missions to establish safe corridors for civilians to escape ferocious Russian shelling in Mariupol, although the Red Cross said its team will try again Saturday.
After five weeks of a military campaign that has reduced parts of Ukraine to rubble, Moscow said this week it would scale back attacks on the capital Kyiv and the city of Chernigiv.
But Ukrainian President Volodymyr Zelensky said Russia was consolidating and preparing "powerful strikes" in the east and south, joining a chorus of Western assessments that Moscow troops were regrouping, not withdrawing.
Zelensky on Friday played host to European Parliament President Roberta Metsola in Kyiv, hailing her "heroism" for visiting the war zone.
"We are glad that you are on the side of the light and the good," Zelensky told Metsola.
"Courage, strength, resolve," Metsola said on Twitter, posting a photograph of her and Zelensky shaking hands.
The airstrike in Russia hit energy giant Rosneft's fuel storage facility in the western town of Belgorod, around 40 kilometres (25 miles) from the border with Ukraine.
Oleksiy Arestovych, an aide to Ukraine's president, said in a Twitter video that "for what's happening on Russia's territory, the responsibility lies with Russia, and it's up to them to deal with."
But the consequence on peace negotiations was swiftly made clear by Moscow.
"This is not something that can be perceived as creating comfortable conditions for the continuation of negotiations," Kremlin spokesman Dmitry Peskov told reporters.
Russia launched its invasion on February 24, expecting to quickly take Kyiv and topple Zelensky's government.
A ferocious Ukrainian fightback and Russia's logistics and tactical problems scuppered such plans, with Russia also battling unprecedented Western sanctions that have led multinationals to quit the country en masse.
US officials Friday gave a grim assessment of Russia's economy, warning it will tumble into a "deep" recession and shrink by 10 percent.
On the ground, Ukraine's troops were beginning to reassert 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 defence ministry.
Civilians have trickled out of devastated areas as Ukrainian forces liberated areas around Kyiv and Chernigiv.
Three-year-old Karolina Tkachenko and her family had walked an hour through a field strewn with burnt-out Russian armoured vehicles to flee their village outside Kyiv.
"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 Karolina's mother Karina Tkachenko.
"I hope all this will end soon, and I will go back to my work," she told AFP.
In Mariupol, Viktoria Dubovytskaya, who had sheltered in the theatre where 300 people are feared to have been killed in Russian bombardments, said she only grasped the extent of the destruction as she fled.
Bodies lay in the rubble, and small wooden crosses were planted in the ground, she told AFP.
"When people find their loved ones, they just bury them wherever they can. Sometimes where roses used to bloom," she said. "The city is now a common grave." An estimated 160,000 people are still desperately trapped in Mariupol with little water, food and electricity.
The UN's cultural agency said Friday it has confirmed at least 53 Ukrainian historical sites, religious buildings and museums have sustained damage during the invasion.
Ukraine also warned that Russian forces who left Chernobyl nuclear plant -- site of the world's worst nuclear accident, in 1986 -- after weeks of occupation may have been exposed to radiation.
"Russia behaved irresponsibly in Chernobyl" by digging trenches in contaminated areas and keeping plant personnel from performing their duties, said Foreign Minister Kuleba.
With his economy crippled by unprecedented international sanctions, Putin has sought to leverage Russia's status as an energy power.
Warning that EU members will need to set up ruble accounts from Friday to pay for his country's gas, he said existing contracts would be halted if the payments were not made.