The Russian commander behind the atrocities in Bucha has been named and pictured.
It comes as "mounting evidence" of war crimes committed by Russian forces in the city near capital Kyiv will be discussed by a United Nations Security Council today.
The Ukrainian President - who was seen holding back tears as he visited the aftermath on Monday - will address the council, which is pushing to ensure "justice is done".
Volodymyr Zelensky surveyed the alleged atrocities near capital Kyiv, describing the discovery of raped women and murdered children among the dead.
He was accompanied by defending servicemen after invading troops had retreated from the area.
US President Joe Biden called for Kremlin leader Vladimir Putin to face a war crimes trial, adding he is now seeking further sanctions against Russia.
"This guy is brutal and what's happening in Bucha is outrageous and everyone's seen it," he said in an impassioned speech last night.
Ukrainian officials have vowed to hunt down the "butchers of Bucha" after hundreds of dead civilians were discovered.
Kyiv intelligence sources have named Lt Colonel Azatbek Omurbekov - who leads the 64th Separate Motorised Rifle Brigade which occupied the region - as the commander in the area.
Growing evidence points to barbarism in Bucha with local women and children raped and shot and demands are growing for accountability over atrocities by Vladimir Putin's forces.
Zelensky has warned that worse evidence of mass killings of civilians by Moscow will emerge as the Russian president repositions his troops to the south-east.
It will be his first address to the Security Council, whose presidency is held by the UK, with members including Russia and China.
The Ukrainian leader accused Moscow of committing "real genocide", as he appeared visibly emotional, flanked by his soldiers and wearing a bullet-proof vest.
Ihor Zhovkva, deputy head of the office of the President of Ukraine, said Mr Zelensky "will give all the evidence" to the international community at the Security Council meeting.
"If these awful atrocities will not change the agenda of all the world, what will change?" the adviser asked on BBC Radio 4's Today programme.
Mr Zhovkva said Ukraine needs "additional weapons", including tanks and missiles, to "liberate more cities" from Russia control.
Dame Barbara Woodward, Britain's UN ambassador, called images coming from Bucha "harrowing, appalling, probable evidence of war crimes and possibly a genocide".
She cited hundreds of bodies being dumped in the streets or in mass graves and allegations of rape.
Ukrainian reporter Evgeny Spirin said: “Some of the killed children had their arms tied up. Children…. They were tying up children. There were shot dogs."
He added: “Irpen, Bucha, Dmitrievka. How to live with this, and is it worth living at all? How do I explain this to myself, how?
“They must cease to exist. They must all be destroyed, each and every one of them, turned into dust, into mud, into clay, all of them.”
Ukrainian activists and web sleuths have identified the presence of Omurbekov’s forces - which are normally based in the far east of Russia - and a number of other Russian military units.
Volunteers group Tretya Sila reported: "In Bucha there was a military unit 51460 from the village of Knyaze-Volkonskoye, Khabarovsk region...
“Soon all these murderers, rapists and looters will be known by name."
InformNapalm volunteers named Omurbekov as unit 51460 of the 64th Separate Motorized Rifle Brigade amid suspicions of grotesque war crimes in Bucha.
In the city, too, according to the Ukrainians, were forces from the Fifth Separate Guards Tank Brigade of 36th army of the Russian Eastern military district, the 331st Parachute Assault Regiment of 98th airborne division, the 137th Parachute Assault Regiment of 106th airborne division, the 104th and 234th Air Assault Regiments of the 76th Guards Air Assault Division, 14 and 45 separate guard brigades of special purpose; the 63rd special operational regiments of the Russian national guard, and the 155th separate guards brigade of marines of the Pacific Fleet.
So far no commanders have been named from these forces apart from Omurbekov.
Russian sources have hit back claiming the Ukrainians were responsible for the atrocities in Bucha and other locations - and are the ones guilty of war crimes.
Russian ambassador to the US Anatoly Antonov said: “The Russian defence ministry has fully rejected these false accusations.”
He said: ”I would like to point out that Russian troops left Bucha on March 30.
“The Ukrainian authorities remained silent all these days, and now they have suddenly posted sensational footage in order to tarnish Russia’s image and make Russia defend itself.
"I would like to emphasise with full responsibility that not a single civilian suffered from violence when the town was controlled by the Russian Armed Forces.ed Forces.
On the contrary, our troops delivered 452 tonnes of humanitarian aid for civilians.
"Meanwhile, the fact that the Ukrainian Armed Forces shelled the town of Bucha right after Russian troops had left was deliberately ignored in the US.
“This is what could have caused civilian casualties. That said, the Kiev regime is clearly trying to blame its atrocities on Russia.”
Konstantin Kosachev, deputy speaker of the Russian upper house of parliament, said: “There are no doubts that it is a staging organised by Kyiv authorities.
“And the fact that the West has picked it up and is hyping it makes it an accomplice of this cynical and immoral crime.”
Foreign ministry spokeswoman Maria Zakharova claimed Kyiv colluded with the West in seeking to blame Russia.
"I think that the very fact that these statements were made minutes after these materials were released leaves no doubts as for who has contracted this story," she told state-run Rossiya-1 television channel.
The West decided immediately Russia was to blame “based on the videos, seconds-long videos and several photos.
“And they are ready to pour accusations.
“No expert work done, no information from the other side throwing light on what is going on..."
She claimed the massacres were "a provocation staged by the Ukrainian military and radical nationalists” to disrupt peace talks.
Also today, Foreign Secretary Liz Truss will hold talks with her Polish counterpart, Zbigniew Rau, during the second day of her visit to the capital, Warsaw.
She has called for Russia to be hit with the "maximum level" of sanctions and for more weapons to be supplied to Mr Zelensky's forces to help repel the Kremlin's troops.
"The idea that we should wait for something else bad to happen is just completely wrong," she said, applying pressure to allies.
"The reality is that money is still flowing from the West into Putin's war machine - and that has to stop."
She pressed for a "clear timetable" to eliminate imports of Russian oil, gas and gold, and for a further crackdown on industries financing Moscow's invasion.
Poland's prime minister Mateusz Morawiecki has criticised German Chancellor Olaf Scholz for focusing on the "voices of German businesses" rather than the innocents slain in Ukraine.
Prime Minister Boris Johnson is likely to urge Germany to set a date for phasing out Russian gas when he meets Mr Scholz in Downing Street on Friday.
