Former President Mary Robinson has called for a special United Nations tribunal to hold Russia to account on crimes against humanity, particularly in Mariupol.
She told RTÉ's News At One programme that the international community has to end impunity.
Over the weekend, international journalists found bodies, some with their hands bound, in the town of Bucha outside Ukraine's capital after Kyiv's forces retook it from Russia's army.
"It's grim that it's so visible that we can actually have reporters going in, we have satellite imagery and the site secured that’s so important.
"What we are witnessing are the essence of war crimes and crimes against humanity but they have to be proved with very strict evidence," Mrs Robinson said.
International outrage spread yesterday over civilian killings in northern Ukraine, where a mass grave and tied bodies of people shot at close range were found in Bucha, a town taken back from Russian forces, as Moscow shifted the focus of the fighting elsewhere.
The deaths in Bucha, outside Kyiv, drew pledges of further sanctions against Russia from the United States and Europe.
Mrs Robinson explained that as part of her work at the UN High Commission of the Human Rights she understands "about securing the site, getting as much reporting as possible of direct contact and then the forensic examination" and she said that "all of that has to happen".
"The International Criminal Court is seized with the matter now, the prosecutor has agreed and it has been supported by 39 countries including Ireland that they will look at war crimes and crimes against humanity.
"Crimes against humanity is more than an individual war crime, such as a murder or torture or rape of civilians or taking hostages that violates the laws of war.
"Crimes against humanity are certain acts that are purposefully taken as part of a widespread or systematic policy directed against civilians," she said.
Yesterday, a team of the International Committee of the Red Cross was not able to reach Mariupol to evacuate civilians, a spokesperson said, citing security conditions.
Mrs Robinson said that "Mariupol is a prime example of that, the siege, in the way that the Red Cross has been unable to get in, all of that needs to be documented as crimes against humanity as well as war crimes against individual civilians that are hurt".
"The Elders want to go further, and we are calling for a tribunal to be established to prosecute President Putin for potential crime of aggression.
"It exists under the Rome Statute, which is the famous statutes to deal with this kind of area, but unfortunately it can't be used directly as neither Ukraine or Russia is a member of the Statute.
"The only other way would be for it would to be referred by the UN Security Council and Russia would veto that so we need a special tribunal," she said.
