High-resolution satellite imagery has shown bodies have been lying in the open for weeks in the Ukrainian city of Bucha, as Russia denied it had committed war crimes.

Russia’s UN ambassador, Vassily Nebenzia, insisted at a news conference that during the time that Bucha was under Russian control “not a single local person has suffered from any violent action”.

But satellite imagery from commercial provider Maxar Technologies, first reported by The New York Times, proved the bodies had been there for weeks.

Western and Ukrainian leaders have accused Russia of war crimes before, and the International Criminal Court’s prosecutor has already opened an investigation. But the latest reports ratcheted up the condemnation.

US president Joe Biden called for a war crimes trial against Russian leader Vladimir Putin and said he will seek more sanctions after the reported atrocities in Ukraine.

“You saw what happened in Bucha,” Mr Biden said, describing Mr Putin as a “war criminal”.

German foreign minister Annalena Baerbock said photographs from Bucha show the “unbelievable brutality of the Russian leadership and those who follow its propaganda”.

French president Emmanuel Macron said there is “clear evidence of war crimes” in Bucha that demand new punitive measures.

“I’m in favour of a new round of sanctions and in particular on coal and gasoline. We need to act,” Mr Macron said on France-Inter radio.

Though united in outrage, the European allies appeared split on how to respond.

While Poland urged Europe to quickly wean itself off Russian energy, Germany said it would stick with a gradual approach of phasing out coal and oil imports over the next few months.

Russia had withdrawn many of its forces from the capital area after being thwarted in its bid to swiftly capture Kyiv.

It has instead poured troops and mercenaries into the country’s east in a stepped-up bid to gain control of the Donbas, the largely Russian-speaking industrial region that includes the besieged southern port city of Mariupol, which has seen some of the heaviest fighting and worst suffering of the war.

About two-thirds of the Russian troops around Kyiv have left and are either in Belarus or on their way there, probably getting more supplies and reinforcements, said a senior US defence official.

More than 1,500 civilians were able to escape Mariupol on Monday, using the dwindling number of private vehicles available to get out, Ukrainian deputy prime minister Iryna Vereshchuk said.

But amid the fighting, a Red Cross-accompanied convoy of buses that has been thwarted for days on end in a bid to deliver supplies and evacuate residents was again unable to get inside the city, Ms Vereshchuk said.

Ukrainian president Volodymyr Zelensky will speak on Tuesday to UN Security Council diplomats outraged by growing evidence that Russian forces deliberately killed civilians.

Mr Zelensky, speaking from Ukraine, planned to address the most powerful UN body after it receives briefings from secretary-general Antonio Guterres, his political chief Rosemary DiCarlo and UN humanitarian chief Martin Griffiths, who is trying to arrange a ceasefire.

Mr Griffiths met Russian officials in Moscow on Monday and is due to visit Ukraine.

After touring neighbourhoods of Bucha and speaking to hungry survivors lining up for bread, Mr Zelensky pledged in a video address that Ukraine would work with the European Union and the International Criminal Court to identify Russian fighters involved in any atrocities.

“The time will come when every Russian will learn the whole truth about who among their fellow citizens killed, who gave orders, who turned a blind eye to the murders,” he said.

Meanwhile, the Ukrainian military said Russian forces were preparing for an offensive in the south-east of Ukraine.

Russian forces are focused on seizing the cities of Popasna and Rubizhne in the Donetsk and Luhansk regions and the Black Sea port of Mariupol, the general staff said on its Facebook page.

Donetsk and Luhansk are controlled by Russian-backed separatists and recognised by Moscow as independent states.

The general staff said access to Kharkiv in the east, Ukraine’s second-largest city, was blocked.

“The enemy is regrouping troops and concentrating its efforts on preparing an offensive operation in the east of our country,” the statement said.

“The goal is to establish full control over the territory of Donetsk and Luhansk regions.”