The prime minister said “no denial or disinformation from the Kremlin” can conceal that president Vladimir Putin is “desperate” and “his invasion is failing”.
His comments came after images emerged of dead civilians found on the streets of the small city of Bucha and the Kyiv suburb Irpin, in what an adviser to Ukrainian president Volodymyr Zelensky said resembled a “horror movie”.
An adviser to Ukrainian president 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 Kyiv, watched as Ukrainian soldiers used cables to drag bodies off a street – from a distance due to fears they may be booby-trapped – as Russian troops continued to withdraw and focus attacks on other parts of Ukraine.
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 said: “Russia’s despicable attacks against innocent civilians in Irpin and Bucha are yet more evidence that Putin and his army are committing war crimes in Ukraine.
“No denial or disinformation from the Kremlin can hide what we all know to be the truth – Putin is desperate, his invasion is failing, and Ukraine’s resolve has never been stronger.
“I will do everything in my power to starve Putin’s war machine. We are stepping up our sanctions and military support, as well as bolstering our humanitarian support package to help those in need on the ground.
“The UK has been at the forefront of supporting the International Criminal Court’s (ICC)’s investigation into atrocities committed in Ukraine, and the justice secretary has authorised additional financial support and the deployment of specialist investigators – we will not rest until justice is served.”
The Ministry of Justice last week announced an additional £1 million in funding would be provided to the ICC, and soldiers with military expertise would be assigned to help uncover evidence.
Scotland Yard’s War Crimes Team would also be mobilised to assist the investigation, it added.
Foreign secretary Liz Truss said “indiscriminate” attacks by Russian forces against innocent civilians in Irpin and Bucha must be investigated as war crimes.
She said the UK would “not rest” until those responsible for “atrocities” in Ukraine had faced justice, adding that Russia would not be allowed to cover up its involvement through “cynical disinformation”.
Mr Zelensky’s spokesman Sergey Nikiforov said authorities in Ukraine had found what looked “exactly like war crimes”, including the bodies of executed civilians and mass graves.
Mr Nikiforov told the BBC’s Sunday Morning programme the scenes discovered in de-occupied territories such as Bucha were “really hard to describe”.
He said: “We found mass graves. We found people with their hands and with their legs tied up… and with shots, bullet holes, in the back of their head.
“They were clearly civilians and they were executed.
“We found half-burned bodies as if somebody tried to hide their crimes but they didn’t have enough time to do it properly.”
Russia’s defence ministry has denied accusations of Russian troops killing civilians in Bucha.
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Additional reporting by Press Association