Ukrainian troops are reportedly finding brutalised bodies and widespread destruction in suburbs of the capital as Russian soldiers withdraw and Moscow focuses its attacks elsewhere.
Associated Press journalists in Bucha, a small city northwest of Kyiv, saw the bodies of at least nine people in civilian clothes who appeared to have been killed at close range.
At least two had their hands tied behind their backs. The reporters also saw two bodies wrapped in plastic, bound with tape and thrown into a ditch.
Authorities said they were documenting evidence as Ukraine's military reclaims territory and discovers indications of execution-style killings to add to their case for prosecuting Russian officials for war crimes.
Oleksiy Arestovych, an adviser to Ukrainian president Volodymyr Zelensky, said scores of civilians who had been killed were found on the streets of Bucha and the Kyiv suburbs of Irpin and Hostomel in what looked like a "scene from a horror movie".
Mr Arestovych said some people were shot in the head and had their hands bound, and some bodies showed signs of torture, rape and burning.
The capital city's mayor, Vitali Klitschko, said civilians were "shot with joined hands" and told German newspaper Bild that "what happened in Bucha and other suburbs of Kyiv can only be described as genocide".
A day earlier, AP journalists witnessed Ukrainian soldiers removing at least six bodies from a street in Bucha with cables in case the Russians had booby-trapped corpses with explosives before their withdrawal.
Local residents said the dead people were civilians killed without provocation, a claim that could not be independently verified.
Meanwhile, UK Foreign Secretary Liz Truss said "indiscriminate attacks" by Russian forces against Ukrainian civilians should be investigated as war crimes.
Watch: How Russia are using drones against Ukraine.
She said: "As Russian troops are forced into retreat, we are seeing increasing evidence of appalling acts by the invading forces in towns such as Irpin and Bucha.
"Their indiscriminate attacks against innocent civilians during Russia's illegal and unjustified invasion of Ukraine must be investigated as war crimes.
"We will not allow Russia to cover up their involvement in these atrocities through cynical disinformation and will ensure that the reality of Russia's actions are brought to light."
Liz Truss said the UK will "not rest" until those responsible for "atrocities" in Ukraine have faced justice.
She said: "The UK will fully support any investigations by the International Criminal Court, in its role as the primary institution with the mandate to investigate and prosecute war crimes."
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