Russian president Vladimir Putin should be put on trial at a special international tribunal set up to investigate alleged war crimes in Ukraine, a European Court of Human Rights judge has said.

Ukrainian judge Dr Mykola Gnatovsky, who joined the ECHR in July, said the establishment of a special tribunal is needed as the scale of what has happened in his country has not been seen since World War II.

Speaking to RTÉ's News At One programme before giving the University of Galway's annual human rights lecture later today, Dr Gnatovsky said existing national and international courts do not have the tools to fully examine the alleged crimes.

And, calling for those directly responsible for the start of the war in Ukraine to face trial, he said senior Kremlin officials including Mr Putin should be held accountable for any crimes which have been committed.

"War crimes are under the jurisdiction of the Ukrainian court and the International Criminal Court in the Hague, whereas the crime of aggression - the mother of crimes so to say - the criminal decision to wage a criminal war against Ukraine, is not unfortunately," he said.

"That's why the architecture of these international institutions has this gap which has to be filled and this tribunal is meant to fill this gap.

"This [putting Vladimir Putin on trial at a special criminal tribunal] has to happen, the people responsible for this crime they have to be held to account because our world has not seen a similar war since the end of the Second World War.

"Now, as to how realistic it is, well in 1942 when the St James's Declaration was signed calling for the prosecution of nazis over the crimes they were committing in the Second World War, no one actually knew when the tribunal would be taking place.

"It took three years for the Nuremburg tribunal to materialise, so this is something that has to take place at this stage and there is no time to lose."

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The possibility of a special international tribunal into Russia's war in Ukraine was first raised in March of this year when Dr Gnatovsky and dozens of other global legal and political figures signed a public declaration calling for its establishment.

While the Council of Europe and European Parliament has voiced its support for the move, the United Nationals General Assembly is still discussing what should happen next.

Meanwhile, Dr Gnatovsky also told RTE's News At One programme he believes legitimate allegations of war crimes by Ukrainian soldiers against their Russian counterparts should also be investigated by any potential tribunal "in the most thorough and effective manner".

"Ukraine is a party to the European Convention on Human Rights, and any information that gives rise to allegations of killings, of torture, of other forms of ill-treatment, must be effectively investigated," he said.

Asked about the decision by Ukraine's government to put some Russian soldiers on trial within the country - a situation being mirrored in Russia - Dr Gnatovsky said:

"When such a state, such as Ukraine, has a territorial jurisdiction on the crimes committed, it [Ukraine] is not only in a position to do this but under a strict obligation to discharge its duties and obligations here.

"Only when the territorial state is not capable of doing this or is refusing to do that should the international court take up the case.

But in doing this, the Ukrainian court must meet the obligations of fair trials."