The bodies of at least 20 civilian men were found strewn across a street in the town of Bucha, northwest of Kyiv, following the withdrawal of Russian forces from the area, according to disturbing images released by AFP on Saturday.
European leaders have condemned the alleged atrocities and called for an investigation into the Russian military. The Russian Ministry of Defense denied the allegations on Sunday.
Here's a look at how leaders from around the continent are reacting:
“As Russian troops are forced into retreat, we are seeing increasing evidence of appalling acts by the invading forces in towns such as Irpin and Bucha," Truss said in a statement on Sunday.
Their indiscriminate attacks against innocent civilians during Russia’s illegal and unjustified invasion of Ukraine must be investigated as war crimes," Truss added.
Truss stressed that Russia should not be allowed to "cover up their involvement in these atrocities through cynical disinformation," adding that the UK will play its part in ensuring this doesn't occur.
UK Prime Minister Boris Johnson also released a statement on Sunday condemning the events in Ukraine.
“Russia’s despicable attacks against innocent civilians in Irpin and Bucha are yet more evidence that Putin and his army are committing war crimes in Ukraine," the statement said.
“No denial or disinformation from the Kremlin can hide what we all know to be the truth – Putin is desperate, his invasion is failing, and Ukraine’s resolve has never been stronger," the leader added.
"I will do everything in my power to starve Putin’s war machine. We are stepping up our sanctions and military support, as well as bolstering our humanitarian support package to help those in need on the ground," the statement said.
Truss committed the UK's full support to "any investigations by the International Criminal Court (ICC) in its role as the primary institution with the mandate to investigate and prosecute war crimes." On March 24, the UK offered the ICC an additional £1 million funding to assist efforts to investigate Russian war crimes.
Last week, the Attorney General for England and Wales Suella Braverman offered the assistance of top UK lawyer Howard Morrison QC to the Ukrainian Prosecutor General.
She said in a statement that he would "provide independent and expert legal advice to the Ukrainian Prosecutor General Iryna Venediktova in relation to the investigation and prosecution of war crimes committed by Russia in Ukraine."
European Union: Ursula von der Leyen, the President of the European Commission, tweeted on Sunday: "Appalled by reports of unspeakable horrors in areas from which Russia is withdrawing. An independent investigation is urgently needed."
“Perpetrators of war crimes will be held accountable," she continued.
European Parliament chief Roberta Metsola, who became the first leader of a European Union institution to visit Ukraine since the Russian invasion began when she paid a visit to Kyiv on Friday, said the images from Bucha and other liberated areas in Ukraine show the "cold reality of Putin's war crimes."
In a tweet Sunday, Metsola said she was "appalled" by the "atrocities of Russian army in Bucha & other liberated areas" and stressed that the world must be made "aware of what is happening" in Ukraine and "tougher sanctions must be imposed" on Russia in retaliation.
"The perpetrators & their commanders must be brought to justice," she concluded.
Germany: German chancellor Olaf Scholz called the Bucha images “terrible and horrifying" and said the perpetrators must be held accountable.
"We must relentlessly investigate these crimes committed by the Russian military. I demand that international organizations such as the International Committee of the Red Cross be given access to these areas in order to independently document the atrocities. The perpetrators and those who commissioned them must be held consistently accountable," Scholz said.
German foreign minister Annalena Baerbock, meanwhile, said the images coming out of the Ukrainian town of Bucha were “unbearable.”
Baerbock tweeted on Sunday that “Putin's unrestrained violence wipes out innocent families and knows no boundaries," and called for those responsible for “war crimes” to be held accountable.
“We will tighten the sanctions against Russia, and will support Ukraine even more with their defense,” she tweeted.
France: French President Emmanuel Macron also called the images of Bucha "unbearable."
“The images that reach us from Bucha, a liberated city near Kyiv, are unbearable. In the streets, hundreds of civilians were cowardly murdered. My compassion for the victims, my solidarity with the Ukrainians," Macron said in a tweet.
"The Russian authorities will have to answer for these crimes,” he added.
French Foreign Minister Jean-Yves Le Drian warned that if what he called “massive abuses” in Bucha, Ukraine turn out to be war crimes, those responsible will be “tried and convicted.”
“The strongest possible economic and international pressure must be maintained and reinforced on Russia to force the Russian authorities to put an end to the war of aggression that they launched on February 24 against Ukraine, the human cost and humanitarian impact of which are becoming more serious every day," Le Drian said in a statement Sunday afternoon.
Italy: Prime Minister Mario Draghi of Italy on Sunday condemned Russia's alleged violence against civilians in Bucha.
“The images of the crimes committed in Bucha and in the other areas liberated by the Ukrainian army leave us astonished," he said in a statement released by his press office.
“The cruelty of the massacres of unarmed civilians is frightening and unbearable. The Russian authorities must immediately cease hostilities, stop the violence against civilians, and must account for what has happened," he added.
Spain: Spanish Prime Minister Pedro Sánchez expressed “horror, pain, and outrage” after seeing images of civilians killed in the Ukrainian town of Bucha, in a tweet on his official account on Sunday.
The war crimes that are being committed cannot go unpunished,” Sánchez said. “All our solidarity, help and support to the Ukrainian people.”
CNN's Nathan Hodge, Max Foster, James Frater, Stephanie Halasz, Niamh Kennedy, Nicola Ruotolo and Amy Cassidy contributed reporting to this post.