Ukrainian authorities were investigating possible war crimes by Russia after finding hundreds of bodies found around towns near Kyiv.
In the town of Bucha, 37 km northwest of Kyiv's city centre, Reuters reporters saw a man lay sprawled by the roadside with a bullet wound to his head.
Bucha's deputy mayor, Taras Shapravskyi, said 50 of some 300 bodies, found after Russian forces withdrew from the city late last week, were the victims of extra-judicial killings carried out by Russian troops.
Reuters could not independently verify those figures or who was responsible for the killings.
Russia's defence ministry said in a statement issued on Sunday that all photographs and videos published by the Ukrainian authorities alleging "crimes" by Russian troops in Bucha were a "provocation," and no resident of Bucha suffered violence at the hands of Russian troops.
Satellite images showed a 45-foot-long trench dug into the grounds of a Ukrainian church where a mass grave was found this week.
Pictures of the destruction and apparent violence towards civilians sparked widespread condemnation of Russia and leader Vladimir Putin. US Secretary of State Antony Blinken described the images as "a punch in the gut," while United Nations Secretary-General Antonio Guterres called for an independent investigation.
"Putin and his supporters will feel the consequences," said German Chancellor Olaf Scholz, adding that Western allies would agree on further sanctions in the coming days.
Japan said it would consult with allies about additional sanctions.
"Japan takes deaths of innocent civilians in Ukraine extremely seriously. We are really shocked," Chief Cabinet Secretary Hirokazu Matsuno told a regular news conference.
Germany's Defence Minister Christine Lambrecht said the European Union must discuss banning the import of Russian gas - a departure from Berlin's prior resistance to the idea of an embargo on Russian energy imports.
The UN Security Council will discuss Ukraine on Tuesday - as scheduled - and will not meet on Monday as requested by Russia, said Britain's mission to the United Nations, which holds the presidency of the 15-member council for April.
Russia had requested the Security Council convene on Monday to discuss what it called a "provocation by Ukrainian radicals" in the town of Bucha after Kyiv accused Russian troops of killing civilians there.
Russia has previously denied targetting civilians and has rejected allegations of war crimes in what it calls a "special military operation" aimed at demilitarising Ukraine. Ukraine says it was invaded without provocation.
Human Rights Watch said it had documented "several cases of Russian military forces committing laws-of-war violations" in the Ukrainian regions of Chernihiv, Kharkiv and Kyiv.
Ukraine's foreign minister called on the International Criminal Court to collect evidence of what he called Russian war crimes. The foreign ministers of France and Britain said their countries would support any such probe.
However, legal experts say a prosecution of Putin or other Russian leaders would face high hurdles and could take years.