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.

United States President Joe Biden called for a war crimes trial against Russia's President Vladimir Putin and Washington will ask the United Nations 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 UN Security Council on Tuesday proving its forces were not involved. 

In an early morning video address, Zelensky said he would also 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, Zelensky 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. 

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”.

Kuleba also had a call with his Chinese counterpart Wang Yi on Monday, with Beijing again calling for talks to end the conflict. 

The call, which Beijing said was made at Ukraine's request, was the first reported high-level conversation between the countries since March 1, when, according to Kyiv, Kuleba asked Beijing to use its ties with Moscow to stop Russia's invasion.

‘Feel the consequences’

Russia launched what it calls a "special military operation" in Ukraine on Feb. 24, saying it aimed at demilitarizing and "de-Nazifying" its neighbor. Ukraine and the West say the invasion was illegal and unjustified.

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 the coming days.

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

Russia supplies about a third of Europe's gas, and Putin has tried to use energy as a lever to fight back against Western sanctions. So far, however, Moscow has kept its gas flowing into Europe, despite uncertainty over Putin's demands for payments in rubles.