Taras Shapravskyi, deputy mayor of Bucha, a town around 40km northwest of Kyiv city, said 50 of some 300 bodies found after Kremlin forces withdrew late last week were the victims of extra-judicial killings carried out by Russian troops.

Ukrainian authorities said they were investigating possible war crimes there, a description also used by French President Emmanuel Macron. The Kremlin categorically denied any accusations related to the murder of civilians in the town.

Reuters reporters in Bucha saw one man sprawled by the roadside, his hands tied behind his back and a bullet wound to his head. A mass grave at a church remained open, with hands and feet poking through red clay heaped on top.

Pictures of the destruction and civilian deaths in Bucha looked set to galvanise the United States and Europe into fresh sanctions against Moscow.

The images were also poised to overshadow peace talks between Russia and Ukraine, due to restart by video link on Monday against a backdrop of artillery bombardments in Ukraine's south and east, where Russia says it is now focusing its operations after withdrawing from around Kyiv.

Ukrainian television showed smoking fuel tanks and fire trucks near the Black Sea port of Odesa. Russia said it had destroyed an oil refinery used by the Ukrainian military.

Ukraine says it has evacuated thousands of civilians in recent days from the besieged and wrecked port city of Mariupol, a main target for Russia's current offensive and surrounded by areas in the hands of Russian-backed separatists in the Donbas region.

Ukraine was preparing for what its general staff said were about 60,000 Russian reservists called in to reinforce the offensive there, while British military intelligence also said Russian troops, including contractors from the Wagner private military company, were moving to the east.

German Chancellor Olaf Scholz said Russian President Vladimir Putin and his supporters would "feel the consequences" of events in Bucha.

Western allies would agree on further sanctions against Moscow in coming days, he said, though it was not clear if they would included Russian energy exports.

Germany's Defence Minister Christine Lambrecht said the European Union must discuss banning Russian gas - a departure from Berlin's prior resistance to that idea.

France's Macron said new sanctions were needed, including on oil and coal, and there were very "clear clues pointing to war crimes" by Russian forces.

US Secretary of State Antony Blinken described the images from Bucha as "a punch in the gut," while United Nations Secretary-General Antonio Guterres called for an independent investigation.

The UN Security Council will discuss Ukraine on Tuesday but will not meet on Monday as requested by Russia.

Moscow has said killings in Bucha there were "staged" to sully Russia's name, and Kremlin spokesperson Dmitry Peskov said on Monday that its diplomats would press on with efforts to convene a Security Council meeting to discuss what Moscow has called "Ukrainian provocations" in the town.

The facts and chronology did not support Ukraine's version of events there, Peskov told reporters, urging international leaders not to rush to judgment.

Russia has denied targeting civilians and has rejected allegations of war crimes.

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.

Shelling hit the eastern city of Kharkiv on Sunday causing seven deaths and dozens of injuries, local prosecutors said.

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.

Australian Associated Press