KYIV, Ukraine, April 2, 2022 (BSS/AFP) - Ukraine's president refused Friday to say whether he had ordered an airstrike on Russian soil, as a bus convoy navigated a tortuous evacuation to help thousands flee the besieged city of Mariupol.

   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 President Volodymyr Zelensky telling US network Fox News: "I'm sorry, I do not discuss any of my orders as commander in chief."

   With the prospect of war expanding across Ukraine's borders, progress appeared stalled in one of the country's most pressing humanitarian disasters, in the shattered southern city of Mariupol.

   But late Friday people who managed to flee Mariupol to Russian-occupied Berdiansk were from there carried on dozens of buses to Zaporizhzhia, some 200 kilometers (120 miles) to the northwest, according to an AFP reporter on the scene.

   "I am just crying. I just saw my granddaughter," said Olga, who was waiting for relatives at a centre for displaced people in Zaporizhzhia.

   "Her mother's family are still in Mariupol and we don't know if they are alive."

   The evacuation of 3,071 people, according to figures announced by Zelensky, escaping the ferocious Russian shelling of Mariupol, was a rare success in a city that has faced weeks of bombardment.

   At least 5,000 residents have been killed, according to local authorities, and the estimated 160,000 who remain face shortages of food, water and electricity.

  The international Red Cross said a team heading to the city to conduct a separate evacuation effort was forced to turn back Friday after "arrangements and conditions made it impossible to proceed".

   The Red Cross said its team will try again Saturday.

   - Russia regrouping? -

   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 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.

   On Friday he 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.

   Kyiv has grown impatient over the West's reluctance to step up and provide greater military support to Ukraine, with Washington and other capitals concerned about an escalating conflict with nuclear-armed Russia.

   "Just give us missiles. Give us airplanes," Zelensky pleaded on Fox. "You cannot give us F-18 or F-19 or whatever you have? Give us the old Soviet planes. That's all... Give me something to defend my country with."

   - 'Common grave' -

   Ukraine's defence ministry meanwhile said Russian troops were continuing their "partial retreat" from the north of Kyiv towards the Belarusian border.

   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."

   - Radiation risks -

   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 Dmytro Kuleba.