Boris Johnson has condemned Russia’s "despicable attacks" against Ukrainian civilians in Irpin and Bucha, adding that "we will not rest until justice is served".
The Prime Minister said "no denial or disinformation from the Kremlin" can conceal that President Vladimir Putin is "desperate" and "his invasion is failing".
An adviser to Ukrainian president Volodymyr Zelensky earlier said dead civilians had been found on the streets of the small city of Bucha and the Kyiv suburb of Irpin, in what resembled a "horror movie".
The Associated Press said its journalists in Bucha, northwest of the capital Kyiv, watched as Ukrainian soldiers used cables to drag bodies off a street – from a distance due to fears they may be booby-trapped.
In a statement, Mr Johnson said the UK is "stepping up" its sanctions and military support, and "bolstering" humanitarian help for those on the ground.
He added: "The UK has been at the forefront of supporting the International Criminal Court's investigation into atrocities committed in Ukraine.
"The Justice Secretary has authorised additional financial support and the deployment of specialist investigators – we will not rest until justice is served."
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