Ukrainian authorities were investigating possible war crimes by Russia after finding hundreds of bodies, some bound and shot at close range, strewn around towns near Kyiv after Kremlin forces withdrew to refocus their attacks in other parts of the country.
In the town of Bucha, 37 km (23 miles) northwest of Kyiv's city center, Reuters reporters saw a man lay sprawled by the roadside, his hands tied behind his back and 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 defense 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. Reuters reporters in Bucha visited a mass grave at one church that was still open, with hands and feet poking through the red clay heaped on top.
Pictures of the destruction and apparent violence towards civilians sparked widespread condemnation of Russia and leader Vladimir Putin. U.S. 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 Defense 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 U.N. 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 targeting civilians and has rejected allegations of war crimes in what it calls a "special military operation" aimed at demilitarizing and "denazifying" 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.
Russia has pulled back forces that had threatened Kyiv from the north, saying it intends to focus on eastern Ukraine.
Ukraine's General Staff said Russia expected to mobilize about 60,000 reservists.
"The military-political leadership of the Russian Federation has begun measures to covertly mobilize reservists in order to bring military units to wartime status," the general staff said on Monday.
Reuters could not independently confirm the claim.
Explosions were heard in the early hours of Monday in the cities of Kherson and Odesa, in the south, while air raid sirens sounded across the country's east.
British military intelligence said Russian troops, including mercenaries from the state-linked Wagner private military company, are being moved into the east.
Serhiy Gaidai, the governor of eastern Luhansk region, said Russia was building up forces to break through Ukrainian defences.
"I am urging residents to evacuate. The enemy will not stop, it will destroy everything in its path," he said in comments carried on Ukrainian television.
Ukrainian President Volodymyr Zelenskiy appeared in a video aired at the Grammy Awards in the United States and appealed to viewers to support Ukrainians "in any way you can".
"What is more opposite to music? The silence of ruined cities and killed people," said Zelenskiy.
Missiles struck near Odesa on Sunday, with Russia saying it had destroyed an oil refinery used by the Ukrainian military. The Odesa city council said "critical infrastructure facilities" were hit.
Ukraine evacuated more than 2,600 people from the southeastern port of Mariupol and the region of Luhansk on Sunday, Ukrainian Deputy Prime Minister Iryna Vereshchuk said. Ukrainian officials were in talks with Russia to allow several Red Cross buses to enter Mariupol, she added.
The Red Cross abandoned earlier attempts due to security concerns. Russia blamed the charity for the delays.
There was little sign of a breakthrough in efforts to negotiate an end to the war, although Russia's chief negotiator, Vladimir Medinsky, said talks were due to resume on Monday via videoconference.