There are calls for Vladimir Putin to be prosecuted for war crimes after hundreds of dead civilians were found in and around the city of Bucha.
The bodies were found lying in the streets and buried in mass graves as Ukrainian forces retook the territory from retreating Russian troops.
The small city near Kyiv is now in the international spotlight, with US President Joe Biden saying Vladimir Putin should be put on trial for war crimes.
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Here's what we know.
Zelenskyy says Bucha was the site of 'genocide'
Ukraine has accused Russian forces of carrying out a massacre in areas around Kyiv after at least 410 bodies were found.
Some victims had been shot at close range after having their hands bound, and children were among the dead, according to Ukraine.
"Their hands were tied behind their back and they were shot in the back of the head or in the eyes right on the streets," Ukraine's President Volodymyr Zelenskyy said in a video address.
"Civilian cars with people were driven over. Women and girls were raped. And the investigation has just started. We haven't yet collected all the testimonies. We haven't yet located all the burials, searched all the basements where Russians were torturing people."
His address featured footage that showed areas strewn with dead bodies
Mr Zelenskyy said what had happened in Bucha and other areas around Kyiv amounted to "genocide".
The President visited the area on Monday to meet with survivors and survey the damage.
'Tortured before they were killed'
Associated Press journalists saw dozens of bodies in Bucha.
Some were wrapped in black plastic and piled in one end of a mass grave in the grounds of Bucha's Church of St Andrew Pyervozvannoho All Saints.
Many of those buried there had been shot in cars or killed in explosions trying to flee the city, Father Andrii Galavin said. With the morgue full and the cemetery impossible to reach, he said the churchyard was the only place to keep the dead.
Bucha's deputy mayor said on Sunday that 50 residents had been victims of extrajudicial killings carried out by Russian troops.
Ukrainian officials took journalists to the basement of what they said was a summer home for children and showed them the bodies of five men with their hands tied behind their backs.
The officials said the men, who all wore civilian clothes, had been killed by Russian soldiers.
"They were shot, shot either in their head or in their chest. They were tortured before they were killed," said Anton Herashchenko, an adviser to the Ukrainian interior ministry.
He said Russian soldiers had set up camp inside the building and stayed there for three weeks.
The Reuters news agency which spoke to him could not independently verify his account.
There are reports hundreds of people in Bucha were killed by Chechen fighters allied to Russian forces.
Bucha was once a leafy home to 28,000 Ukrainians
Bucha was the scene of heavy fighting early in the war as Russian forces advanced on Kyiv from the north-west.
Normally a leafy town home to about 28,000 people, parts of it have now been reduced to rubble.
Those sheltering in Bucha have had no electricity, water or gas since the beginning of the invasion.
A satellite image captured on March 31, while the city was still in Russian hands, showed a trench around 14 metres in length dug into the grounds of the Church of St Andrew Pyervozvannoho All Saints, US company Maxar Technologies said.
A previous image from March 10 showed early signs of excavation at the same site.
Australia backs claims of Russian war crimes
US Secretary of State Antony Blinken described images of dead bodies as "a punch in the gut".
Australia's Defence Minister Peter Dutton described Russian military action in Ukraine more broadly as "straight up and down the act of a war criminal" who must be held accountable.
Foreign Minister Marise Payne also supported Mr Biden's call for an investigation.
Britain's UN ambassador Barbara Woodward called the images from Bucha "harrowing, appalling, probable evidence of war crimes and possibly a genocide".
Russia strongly denies deliberately killing civilians
Russia's defence ministry denied allegations its forces were responsible for deliberately killing civilians, and accused Ukraine of faking evidence.
Moscow asked the United Nations Security Council to convene to discuss what it called a "provocation by Ukrainian radicals" in Bucha.
Russia's ambassador to the United Nations denied Russian forces committed war crimes.
"This is nothing else but yet another staged provocation aimed at discrediting and dehumanising of the Russian military and levelling political pressure on Russia," ambassador Vassily Nebenzia told reporters.
Zelenskyy says more massacres could be uncovered
Mr Zelenskyy predicts further, worse instances of deliberate killings of civilians by Russian forces will be discovered as Ukraine retakes territory in the north of the country.
"This is only one town — one of many Ukrainian communities which the Russian forces managed to capture," he said ahead of his address to the UN Security Council on Tuesday.
Russian forces have been withdrawing from around Kyiv ahead of what Moscow says will be renewed attacks in Ukraine's east.
ABC/wires
