UK Foreign Secretary Liz Truss on Saturday said that Britain was working with other countries to gather evidence on Russian war crimes in Ukraine, amid growing international calls for bringing Russian President Vladimir Putin to justice for atrocities in Ukraine.

“The UK is working with others to collect evidence and support [for the International Criminal Court] war crimes investigation. Those responsible will be held to account,” Truss wrote on Twitter.

“Appalled by atrocities in Bucha and other towns in Ukraine. Reports of Russian forces targeting innocent civilians are abhorrent,” she further said.

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Truss’ comments come on a day when Carla del Ponte, a top former UN prosecutor who oversaw war crimes tribunals for Rwanda and former Yugoslavia, flatly called Putin a “war criminal” and called for the issuance of an international arrest warrant for the Russian President to bring him to justice.

“I hoped never to see mass graves again. These dead people have loved ones who don’t even know what’s become of them. That is unacceptable,” del Ponte said, commenting on reports of Russia’s use of mass graves to bury those killed on the Ukrainian side.

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She went on to say that other war crimes committed by Russian forces in Ukraine include attacks on civilians, the destruction of civilian buildings, as well as the complete demolition of entire Ukrainian villages.

Del Ponte pointed out that investigations into war crimes in Ukraine would be a relatively straightforward matter as Kyiv itself had requested an international probe into the issue.

Finally, she said that once evidence of war crimes is unearthed, “you must go up the chain of command until you reach those who took the decisions,” adding that it was also possible to directly hold Putin accountable.