Evidence of the atrocities in Ukraine may also lead to stiffer economic sanctions on Russia. The Kremlin is yet to comment publicly on the claims of genocide.
Tory MP Tom Tugendhat, chairman of the Commons foreign affairs committee, said: 'This is the most barbaric example yet of Russia taking full advantage of the West's timidity to step up.
'We condemn from afar, but fail to act as these war crimes in this horrendous war clock up. Until we offer Ukraine far greater direct support, Putin's war machine will continue with impunity.'
In another chilling echo of the Second World War, 15 female prisoners of war were returned in a prisoner swap with their heads shaved.
Ukraine's foreign ministry compared the treatment to that of the Nazis in concentration camps. Human Rights Watch has compiled details of war crimes based on survivors' testimonies.
In one incident in Bucha, Russian soldiers forced five men on to their knees, pulled shirts over their heads and shot them in the back of the head.
Another woman in the town said Russian soldiers took her and 40 others to a square to witness a similar scene with four men – although this time just one of them was shot.
A commander then shouted: 'We are here to cleanse you from the dirt.'
In Staryi Bykiv, east of Kyiv, a woman watched Russians round up her son and five other men and murder them in front of her.
Hugh Williamson, of Human Rights Watch, said: 'The cases we documented amount to unspeakable, deliberate cruelty and violence against Ukrainian civilians.
'Rape, murder, and other violent acts against people in the Russian forces' custody should be investigated as war crimes.'
Elsewhere in the country, a woman said a Russian soldier repeatedly raped her at knife point in the Kharkiv region.
And medical staff in Zaporizhzhia have said they have treated young girls from Mariupol with injuries suggesting they have been sexually assaulted.
The UK's ambassador to Ukraine, Melinda Simmons, said: 'Rape is a weapon of war. Though we don't yet know the full extent of its use in Ukraine it's already clear it was part of Russia's arsenal.'
Russia has also been accused by Ukraine of using children as 'human shields' in the village of Novyi Bykiv, close to Chernihiv.