The international community should issue an arrest warrant for Russian President Vladimir Putin, U.N. ex-chief prosecutor Carla Del Ponte, head of the war crimes tribunals for Rwanda and the former Yugoslavia, urged Saturday.

"Putin is a war criminal," Del Ponte told the Swiss newspaper Le Temps in an interview published Saturday.

In interviews given to Swiss media to mark the release of her latest book, the Swiss lawyer who oversaw U.N. investigations in Rwanda and the former Yugoslavia for the International Criminal Court (ICC) said there were clear war crimes being committed in Ukraine.

She said she was particularly shocked by the use of mass graves in Russia's war on Ukraine, which recalls the worst of the wars in the former Yugoslavia.

"I hoped never to see mass graves again," she told the newspaper Blick. "These dead people have loved ones who don’t even know what’s become of them. That is unacceptable."

Other war crimes she identified in Ukraine included attacks on civilians, the destruction of civilian buildings and even the demolishing of entire villages.

She said the investigation in Ukraine would be easier than that in Yugoslavia because the country itself had requested an international probe. The current ICC chief prosecutor, Karim Khan, visited Ukraine last month.

If the ICC finds proof of war crimes, she said, "you must go up the chain of command until you reach those who took the decisions."

She said it would be possible to bring even Putin to account.

"You mustn’t let go, continue to investigation. When the investigation into Slobodan Milosevic began, he was still president of Serbia. Who would have thought then that he would one day be judged? Nobody," she told Blick.

Del Ponte added that investigations should be carried out into possible war crimes committed by both sides, pointing also to reports about the alleged torture of some Russian prisoners of war by Ukrainian forces.