“I am deeply shocked by the images of civilians killed in Bucha, Ukraine,” Guterres said on Twitter.
“It is essential that an independent investigation leads to effective accountability.”
Earlier in the day, the bodies of at least 410 civilians were recovered in the city of Bucha near the Ukrainian capital Kyiv, said Ukraine's prosecutor-general.
On Sunday, the Russian Defense Ministry denied that its troops killed civilians while withdrawing from towns near Kyiv.
Ukrainian Foreign Minister Dmytro Kuleba called on international organizations to send experts to the country as soon as possible to collect evidence of war crimes committed by Russian troops.
On Saturday, Mikhail Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy, said the bodies of people with their hands tied and shot dead by Russian troops "lie in the streets" in Bucha.
