US President Joe Biden called Monday for a "war crimes trial" over alleged atrocities in Bucha and vowed tougher sanctions against Moscow, as Ukraine's leader urged the world to acknowledge a "genocide" by Russian troops near Kyiv.

Western leaders have united in outrage after dozens of bodies were found on the streets and in mass graves when Russian troops retreated from the devastated town near the capital, laying bare the horrors of a 40-day war that has killed thousands.

Bombardments continued Monday including in southern Mykolaiv, where officials said Russian strikes killed 10 civilians and wounded 46, as Kyiv warned that Moscow was shifting its military focus and preparing a "full-scale" attack in the country's east.

With momentum building for a stiffer European Union response beyond already unprecedented sanctions over Russia's invasion, European Commission chief Ursula von der Leyen said the bloc was ready to send investigators to gather evidence of possible war crimes in Bucha.

Ukrainian President Volodymyr Zelensky blames Russian troops for the killings, but the Kremlin has denied responsibility.

"These are war crimes and it will be recognised by the world as genocide," Zelensky said Monday as he visited the town, where the corpses, some with their hands bound behind their backs, were discovered over the weekend.

"We know that thousands of people have been killed and tortured with extremities cut off, women raped, children killed," he told reporters, wearing a bulletproof vest for a rare trip outside Ukraine's capital, and appearing visibly distressed.

While Moscow suggested images of the corpses in Bucha were "fakes," newly released satellite photographs by Maxar Technologies appeared to rebut such claims, and showed that bodies had been on the streets since mid-March when Russian forces controlled the town.

A grisly new discovery, meanwhile, emerged in Bucha, as the bodies of five men were found in a children's sanatorium basement -- described by the Ukrainian prosecutor general's office as unarmed civilians, their hands tied, who were beaten and then killed by Russian soldiers.

Moscow has sought a UN Security Council meeting on what its deputy ambassador to the body called a "heinous provocation of Ukrainian radicals in Bucha."

But Biden did not hesitate to call out Russian President Vladimir Putin over the killings.

"He is a war criminal," Biden told reporters at the White House.

"What's happening to Bucha is outrageous and everyone's seen it," he said, adding: "We have to gather all the details" in order to "have a war crimes trial."

The killings are likely to receive more international attention on Tuesday, when Zelensky addresses the Security Council's meeting on Ukraine, according to British diplomats.

- 'We dug a mass grave' -

While the horror in Bucha appeared undeniable, the scale of the killings is still being pieced together. On Sunday, Ukrainian prosecutor general Iryna Venediktova said 410 civilian bodies had been recovered in the wider Kyiv region after Russian troops withdrew.

In Bucha, AFP on Saturday saw the bodies of at least 22 people in civilian clothes on a single street.

The town's mayor said 280 people were buried in mass graves because they could not be laid to rest in cemeteries that were within firing range. 

Maxar released satellite images it said showed a mass grave located behind a church there.

Before leaving, Russian forces refused to let residents bury the dead, municipal worker Serhii Kaplychnyi told AFP.

Eventually, they were able to retrieve the bodies, he said. "We dug a mass grave with a tractor and buried everyone."

- Mariupol '90 percent destroyed' -

Russia has redoubled its efforts in Ukraine's south and east, including strikes Sunday on the strategic Black Sea port of Odessa, which Moscow said targeted an oil refinery and fuel depots.

The mayor of Mariupol said Monday 90 percent of the southeastern city had been destroyed since being besieged by Russian forces, trapping some 130,000 residents who have not yet fled.

Vadym Boichenko said plans to evacuate remaining residents from Mariupol, where authorities say at least 5,000 residents have died, were paused because of "incessant" bombings. 

Britain's defence ministry confirmed that recent Russian air activity had focused on southeastern Ukraine, and that Mariupol "continues to be subject to intense, indiscriminate strikes."

The White House, meanwhile, said Russia has withdrawn about two thirds of the troops it had around Kyiv and is seeking to redeploy them to eastern and parts of southern Ukraine, in what is being seen as a substantial shift in strategy.

"Russia has tried to subjugate the whole of Ukraine and it has failed. Now it will attempt to bring parts of the country under its rule," National Security Advisor Jake Sullivan said.

The United States said more sanctions on Russia would be announced "this week," while the EU was urgently discussing similar steps.

French President Emmanuel Macron, who spoke about "very clear indications of war crimes," said new sanctions could target the country's oil and coal sectors.

But Germany warned cutting off Russia's supply of gas to Europe was not yet possible, despite several EU countries pressing for the measure as necessary to face down Moscow.

"We have to cut all economic relationship to Russia, but at the moment, it's not possible to cut the gas supplies. We need some time," German Finance Minister Christian Lindner said as he arrived for talks in Luxembourg.

- 'Something terrible is coming' -

In the eastern city of Kramatorsk, women, children and the elderly boarded trains Monday to flee the Donbas region -- out of the path of Russia's advance.

"The rumour is that something terrible is coming," said Svetlana, a volunteer organising the crowd.

Local governor Sergiy Gaiday warned Monday on Telegram that Russian troops were preparing for a major attack in the Lugansk region inside the Donbas, urging a mass evacuation.

Europe's worst conflict in decades, sparked by Russia's invasion on February 24, has killed as many as 20,000 people, according to Ukrainian estimates.

More than 4.2 million Ukrainians have fled the country and about 6.5 million have been internally displaced, UN agencies say.

The UN's top humanitarian envoy Martin Griffiths was expected in Kyiv after arriving in Moscow Sunday in an attempt to halt the fighting.

And peace talks were scheduled to resume by video Monday.