The United States and Europe were planning new sanctions on Tuesday to punish Moscow over civilian killings in Ukraine, and Ukrainian president Volodymyr Zelenskiy warned more deaths were likely to be uncovered in areas seized from Russian invaders.
Russian forces withdrew from towns north of the capital Kyiv last week as it turns its assault to Ukraine's south and east. Ukrainian troops recaptured towns devastated by nearly six weeks of war, including Bucha, where dead civilians lined the streets.
Searing images of a mass grave in Bucha and the bound bodies of people shot at close range drew an international outcry on Monday.
US president Joe Biden called for a war crimes trial against Russia's president Vladimir Putin and the United States will ask the UN General Assembly to suspend Russia from the Human Rights Council.
Russia denied any accusations related to the murder of civilians and said it would present "empirical evidence" to a meeting of the United Nations Security Council on Tuesday proving its forces were not involved.
In an early morning video address, Mr Zelenskiy said he would also address the Security Council on Tuesday as he builds support for an investigation into the killings in Bucha.
"And this is only one town. One of many Ukrainian communities which the Russian forces managed to capture," Mr Zelenskiy said.
"Now, there is information that in Borodyanka and some other liberated Ukrainian towns, the number of casualties of the occupiers may be even much higher," he added, referring to a town 25km west of 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".
Mr Kuleba also spoke with his Chinese counterpart Wang Yi in a phone call on Monday, with Beijing again calling for talks to end the conflict in Ukraine.
The call, which Beijing said was made at Ukraine's request, was the first reported high-level conversation between the countries since March 1st, when Mr Kuleba asked Beijing to use its ties with Moscow to stop Russia's invasion, the Ukrainian foreign ministry said at the time.
'Feel the consequences'
Russia launched what it calls a "special military operation" in Ukraine on February 24th, aiming to demilitarise and "denazify" Ukraine. Ukraine and the West say the invasion was illegal and unjustified.
Russian forces pulled back from the capital Kyiv in the face of unexpectedly lethal and mobile Ukrainian resistance using 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.
German chancellor Olaf Scholz said on Monday that Mr Putin and his supporters would "feel the consequences" of events in Bucha and that Western allies would agree further sanctions against Moscow in the coming days.
Mr Biden's national security adviser, Jake Sullivan, said new US sanctions against Moscow would be announced this week. The US state department said it was supporting an international team of prosecutors and experts to collect and analyse evidence of atrocities.
France and Germany said they would expel Russian diplomats.
Russia would respond in kind and "slam shut the door on Western embassies", Russian ex-president and deputy head of security council Dmitry Medvedev said.
"It will be cheaper for everyone. And then we will end up just looking at each other in no other way than through gunsights."
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.
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. But Moscow has maintained gas flows through key pipeline routes into Europe, despite uncertainty over Mr Putin's demands for payments in roubles.
The United States stopped the Russian government from paying holders of its sovereign debt more than $600 million from reserves held at American banks, in a move meant to ratchet up pressure on Moscow and eat into its holdings of US dollars.
Battles in the east
Ukraine said it was preparing for about 60,000 Russian reservists to be called in to reinforce Moscow's offensive in the east, where Russia's main targets have included the port of Mariupol and Kharkiv, the country's second-largest city.
Ukraine's general staff said Russian forces aimed to fully take over the Donetsk and Luhansk provinces claimed by Russian-backed separatists and encircle a group of Ukrainian forces.
"Russian troops have attacked Mykolayiv with cluster munitions banned by the Geneva convention. Whole blocks of civilian buildings have come under fire, in particular, a children's hospital. There are dead and wounded, including children," the general staff said in a daily update on Tuesday. Reuters could not independently verify the claims.
In Mariupol, a southeastern town on the Azov Sea that has been under siege for weeks, Reuters images showed three bodies in civilian clothes lying in the street, one against a wall sprayed with blood.
A team from the International Committee of the Red Cross (ICRC) was stopped during an attempt to reach Mariupol to evacuate civilians, and is now being held in a nearby town, a spokesperson said on Monday.
West of Mariupol, in the town of Mykolaiv, shelling on Monday killed 10 people, including a child, and injured 46 others, regional administration head Oleksandr Senkevich said. Reuters was not immediately able to verify the report.