Russia's "indiscriminate attacks" on innocent Ukrainian civilians must be investigated as war crimes, UK Foreign Secretary Liz Truss has said.
There was growing evidence of "appalling acts" in the towns of Bucha and Irpin, she said - and Russia must not be allowed to cover them up with "cynical disinformation".
The BBC's Jeremy Bowen said at least 20 dead men, some with their wrists tied, were found on the streets of Bucha.
Russia has denied targeting civilians.
Ukraine says its forces have retaken the entire region around its capital, Kyiv.
But, as Russian troops have retreated from key towns, the toll of weeks of fierce fighting has been exposed.
Authorities in Ukraine have found what "looks exactly like war crimes" including the bodies of executed civilians and mass graves, a spokesman for Ukraine President Volodymyr Zelensky said.
Sergey Nikiforov told the BBC's Sunday Morning show: "We found people with their hands and with their legs tied up... and with shots, bullet holes, in the back of their head.
"They were clearly civilians and they were executed. We found half-burned bodies as if somebody tried to hide their crimes but they didn't have enough time to do it properly."
Truss said the UK would fully support any investigations by the International Criminal Court.
The UK-led effort to expedite and support an investigation into war crimes in Ukraine was "the largest state referral in its history", she added.
"We will not rest until those responsible for atrocities, including military commanders and individuals in the Putin regime, have faced justice," she said.
She said it was essential the international community continued to provide Ukraine with military and humanitarian support - and step up sanctions.
Other nations have also condemned Russian forces' actions. US Secretary of State Antony Blinken described the images of bodies in the streets of Bucha as a "punch in the gut".
EU chief Charles Michel pledged further sanctions on Moscow. "Shocked by haunting images of atrocities committed by Russian army in Kyiv liberated region #BuchaMassacre," the European Council head wrote on Twitter.
Germany's foreign minister Annalena Baerbock denounced President Putin's "uninhibited violence", saying Russia must pay for its "war crimes" in Bucha in the form of more severe sanctions.
