- UN Secretary-General António Guterres said he was "deeply shocked" by evidence of mass killings in Bucha.
- He said it is "essential" to establish an independent investigation to investigate the massacre.
- The EU and Ukraine are working together to investigate war crimes and crimes against humanity.
UN Secretary-General António Guterres called for an independent investigation into the mass killings of Ukrainian civilians in the Kyiv suburb of Bucha.
"I am deeply shocked by the images of civilians killed in Bucha, Ukraine," Guterres said in a statement on Sunday. "It is essential that an independent investigation leads to effective accountability."
Osnat Lubrani, the UN's humanitarian coordinator for Ukraine, responded to a copy of Guterres' statement on Twitter and said Ukrainians "are enduring a living hell for more than a month, thousands of civilians have died."
Nearly 300 Ukrainians were found buried in mass graves in Bucha, which was under Russian occupation until a few days ago when President Vladimir Putin's forces began to reposition away from the Kyiv area.
Satellite images revealed partially excavated mass graves, while journalists and Ukrainian officials recorded corpses throughout the streets.
European Commission President Ursula von der Leyen said in a Monday statement that she spoke with Ukrainian President Volodymyr Zelenskyy about the "dreadful murders" in Bucha, and said the "perpetrators of these heinous crimes must not go unpunished."
Von der Leyen said she established a joint investigation team with Ukraine "to collect evidence and investigate war crimes and crimes against humanity."
Zelenskyy earlier accused Russia of committing genocide in Ukraine, while US Secretary of State Antony Blinken slammed Russian forces for committing "atrocities."
The US in March accused Russia of committing war crimes in Ukraine, calling out "indiscriminate attacks" on civilians.
Meanwhile, the top prosecutor of the International Criminal Court previously said he is investigating potential war crimes committed by Russia in Ukraine.
The latest UN report said it recorded 3,455 civilian casualties — 1,417 people killed and 2,038 people injured — in Ukraine since Russian invasion began on February 24.
It acknowledged, however, that the "actual figures are considerably higher, as the receipt of information from some locations where intense hostilities have been going on has been delayed and many reports are still pending corroboration."