The body of a civilian lies in the street, amid Russia's invasion of Ukraine, in Bucha, in Kyiv region. Reuters

The UN human rights chief said on Monday she was "horrified" by images of dead bodies in Bucha, near the Ukrainian capital, after Russian soldiers retreated, warning of possible war crimes.

"I am horrified by the images of civilians lying dead on the streets and in improvised graves in the town of Bucha in Ukraine," Michelle Bachelet said in a statement.

"Reports emerging from this and other areas raise serious and disturbing questions about possible war crimes, grave breaches of international humanitarian law and serious violations of international human rights law," she said.

On the other hand, Russia's chief investigator on Monday ordered an official examination of what he called a Ukrainian "provocation" after Kyiv accused the Russian military of massacring civilians in the town of Bucha.

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Alexander Bastrykin, head of the Russian Investigative Committee, ordered that a probe be opened on the basis that Ukraine had spread "deliberately false information" about Russian armed forces in Bucha, the committee said in a statement.