Moscow accused the West of staging the deaths to discredit its troops, denied that they had carried out any atrocities and threatened to expel more Western diplomats.

Russia pulled out its forces from towns north of the capital Kyiv last week as it turns its assault to Ukraine's south and east.

As Ukrainian troops recaptured towns devastated by nearly six weeks of war, including Bucha, dead civilians were found lining the streets.

Grim images of a mass grave in Bucha and the bound bodies of people shot at close range prompted an international outcry and pledges of more sanctions against Moscow.

US President Joe Biden called for a war crimes trial against Russia's President Vladimir Putin and Washington will ask the UN General Assembly to suspend Russia from the Human Rights Council.

Dmitry Medvedev, Russia's former president and a close Putin ally who now serves as deputy head of his security council, said the reports of civilian killings in Bucha were "fakes" aimed at discrediting Russia.

Moscow said it would present "empirical evidence" to a meeting of the United Nations Security Council on Tuesday proving its forces were not involved.

Russia's defence ministry said on Tuesday that Ukrainian special services had staged alleged killings of civilians in Ukrainian towns in an attempt to spread propaganda through the Western media.

"Soldiers of the 72nd Ukrainian Main Centre for Psychological Operations conducted another staged filming of civilians allegedly killed by the violent actions of the Russian armed forces" on April 4 in a village 23km northwest of Kyiv, the ministry said.

Zelenskiy said he would speak to the Security Council on Tuesday as he builds support for an investigation into the killings in Bucha and other actions of the Russian military.

In Bucha at least 300 civilians had been killed and the number was likely to get much larger, Zelenskiy said.

"And this is only one town. One of many Ukrainian communities which the Russian forces managed to capture," he said, adding there were reports of more deaths in other liberated towns.

Zelenskiy said Ukraine had no option but to negotiate with Russia to end fighting but that he and Russian President Vladimir Putin might not personally hold talks.

He said the events in Bucha were unforgivable but Ukraine and Russia should take the difficult option of pursuing talks, and signalled that Moscow should recognise what its troops were alleged to have done.

In Bucha Reuters saw several bodies apparently shot at close range, along with makeshift burials and a mass grave in Bucha, but could not independently verify the number of dead or who was responsible.

Ukraine's foreign minister, Dmytro Kuleba, said he spoke with UN Secretary-General Antonio Guterres about Bucha and stressed "that Ukraine will use all available UN mechanisms to collect evidence and hold Russian war criminals to account."

Russian forces pulled back from the capital Kyiv in the face of stiff Ukrainian resistance mounted with the help of Western anti-tank weaponry.

Moscow painted the withdrawal as a goodwill gesture at peace talks, which last convened on Friday. Negotiators had been due to convene on Monday, but neither side has given an update on the talks.

Both Germany and the Biden administration said new sanctions against Moscow were due to be announced in coming days.

The US State Department also said it was supporting an international team of prosecutors and experts in their work collecting and analysing evidence of atrocities.

France and Germany said they would expel Russian diplomats and Medvedev said Moscow would respond in kind and "slam shut the door on Western embassies".

German Defence Minister Christine Lambrecht said the European Union must discuss banning Russian gas, though other officials urged caution around measures that could touch off a European energy crisis.

In the latest effort to pile pressure on Moscow, United States stopped the Russian government from paying holders of its sovereign debt more than $US600 million from reserves held at US banks.

Australian Associated Press