The discovery of bodies scattered along a street in a Ukrainian town left by Russian forces drew widespread condemnation in the West on day 39 of the invasion.
UK Prime Minister Boris Johnson accused Russia of "despicable attacks" on Ukrainian civilians in Bucha and another town, saying his country would not rest until justice was served for the victims.
US Secretary of State Antony Blinken described the images of corpses in Bucha as a "punch to the gut" and the head of the European Commission, Ursula von der Leyen, said an independent investigation was urgently needed.
Ukraine's president, Volodomyr Zelensky, accused Russia of committing genocide.
Russia's defence ministry countered by suggesting the scenes in Bucha had been faked by the Ukrainian government.
Fighting continued, with Russia launching missiles at the Black Sea port of Odesa and other targets in central and southern Ukraine, and reported Russian shelling of the city of Kharkiv.