Australia is demanding a war crimes investigation into an apparent massacre in Ukraine, but an international law expert warns it is "impossible" to predict whether Russian President Vladimir Putin will ever stand trial.
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Kyiv has accused Moscow of "genocide" in Bucha, a small city near Kyiv, after its mayor claimed Russian troops had murdered roughly 50 civilians during a month-long occupation.
Images show the bodies of civilians, many handcuffed, scattered across roads, while survivors say Russian forces committed widespread rape and summary killing of innocents.
Foreign Minister Marise Payne on Tuesday described the scenes as "appalling" and said Australia would support efforts to mount an investigation.
But Senator Payne, who will travel to Brussels for a meeting with NATO leaders this week, stopped short of endorsing the term genocide.
"It's difficult to find the words. It's beyond appalling. It's horrific. It's obscene," she told Channel Seven.
"The bombing of known civilian shelters ... the butchering of people in mass graves, the murder, and the use of rape as a weapon of war, is completely unacceptable."
Senator Payne confirmed two Australian specialists had been offered to the International Criminal Court to assist an investigation.
International law expert at the Australian National University Don Rothwell said high-level Australian Federal Police investigators "have got skills to bear", having been dispatched to Ukraine to investigate the 2014 downing of passenger flight MH-17.
A host of Russians, from foot soldiers to high-ranking politicians, could be subject to international charges for the atrocities in Bucha.
But while Professor Rothwell said an international arrest warrant for Mr Putin was likely, he warned it was impossible to predict whether he would ever face trial.
"There's so many potential wildcards ... if he remains within Russia, and even when he eventually steps down as president has a sympathetic successor regime, he will probably enjoy ongoing protections," he said.
Professor Rothwell said Mr Putin was unlikely to be held responsible for "ordinary war crimes committed on the streets of Ukraine".
"He could possibly be held responsible for a crime against humanity, he could possibly be responsible for genocide. But he would certainly be responsible for a crime of aggression," he said.
"Once you start going up the command chain, the political chain, you're looking at the much bigger crimes against international law."
But Russia is not a signatory to the Rome Statute, a "legal loophole" Professor Rothwell said would make charging Russian authorities with the crime of aggression difficult.
In response, former British Prime Ministers Gordon Brown and John Major are among a host of experts calling for a Nuremberg-style tribunal.
"Effectively it would be the responsibility of the Europeans, the North Americans, Australia and other like-minded countries, to band together to create this body to prosecute for the crime of aggression," Professor Rothwell said.
"The principal target for that would clearly be Putin."
Following Washington formally labelling Mr Putin a war criminal this month, US President Joe Biden has cited the atrocities in Bucha to demand Russian President Vladimir Putin face prosecution at the ICC.
Moscow has accused the west of staging the massacre, but an analysis of satellite images by The New York Times shows bodies sprawled across the street for three weeks - when the town was under Russian control.
During a visit to the site, Ukrainian President Volodymyr Zelensky warned civilians were being "found in barrels, cellars, strangled and simply tortured".
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Mr Zelensky, who urged increased support from Australia during an address to parliament last week, confirmed peace talks were still ongoing, despite what he labelled a "genocide" underway in Ukraine.
The European Union pinned responsibility for the killings on Russia forces, insisting "responsible government officials and military leaders will be held accountable".
"The massacres in the town of Bucha and other Ukrainian towns will be inscribed in the list of atrocities committed on European soil," its statement read.