The EU is preparing a fresh wave of sanctions against Russia after reports and images of apparent executions of unarmed civilians and other atrocities emerged in the wake of Russia’s military retreat from the outskirts of Kyiv.

Charles Michel, president of the European Council, said further sanctions from the bloc “are on their way” in response to Russia’s actions in Bucha.

“Shocked by haunting images of atrocities committed by Russian army in Kyiv liberated region,” Michel tweeted on Sunday. “Further EU sanctions & support are on their way. EU is assisting Ukraine & NGOs in gathering of necessary evidence for pursuit in international courts.”

Liz Truss, British foreign secretary, said she was “appalled by atrocities in Bucha and other towns in Ukraine” as the UK was collecting evidence of war crimes.

The threat of more punitive measures from the west targeting Russia’s economy and its governing elite come as Ukrainian authorities and NGOs have accused Russian forces of perpetrating atrocities against unarmed civilians in areas near Kyiv which have been retaken by Ukrainian forces.

Map showing territory with Russian military presence and territory regained from the Russians by the Ukrainians around Kyiv

Allegations of war crimes have emerged after Moscow started withdrawing its troops from the outskirts of the Ukrainian capital last week, instead shifting its war focus to the country’s east.

Emine Dzheppar, Ukraine’s deputy foreign minister, said soldiers who had retaken Bucha, a small city 60km north-west of the capital, from Russian forces had reported “numerous civilians shot dead”.

“Some of [the] victims have their hands tied. Innocent victims. They didn’t deserve that,” she said.

In the nearby village of Motyzhyn Russian soldiers also “did terrible things”, she added. “Their cruelty is limitless. Before Ukrainian troops arrived, [the] Russian army killed as many civilians as possible. Inhuman. Terrible. Speechless.”

Mykhailo Podolyak, a senior adviser to Ukrainian president Volodymyr Zelensky, posted an image on Twitter appearing to show multiple dead bodies on the streets of Bucha. Some of the victims appeared to have their hands tied behind their backs.

“The bodies of people with tied hands, who were shot dead by [Russian] soldiers lie in the streets,” Podolyak said. “These people were not in the military. They had no weapons. They posed no threat. How many more such cases are happening right now in the occupied territories?”

People who visited the areas or spoke to residents in recent days said they saw corpses of civilians who appeared to have been shot, freshly dug graves, and other indications of suspected war crimes and crimes against humanity, prompting growing calls to investigate and prosecute the Russian officials responsible.

Human Rights Watch said on Sunday that it had documented several cases of unlawful violence that it described as “apparent war crimes”, including in the Chernihiv, Kharkiv, and Kyiv regions.

The New York-based watchdog group said that the cases it documented, which included summary executions and rape, pointed to “unspeakable, deliberate cruelty and violence against Ukrainian civilians”, and said that they should be investigated as war crimes.

Human Rights Watch said that in the village of Staryi Bykiv in Chernihiv region, Russian forces had rounded up six men on February 27, and then executed them.

Gyunduz Mamedov, a former deputy prosecutor-general of Ukraine and specialist in international criminal law who toured Irpin and Bucha north-west of Kyiv at the weekend, said in addition to widespread destruction of infrastructure, he had seen corpses of civilians and freshly dug graves marked with crosses.

He said that about 50 per cent of buildings in Irpin had been damaged and about 300 civilians killed during the Russian offensive.

“Mass graves. Bodies of civilians strewn across streets. Monstrous crimes becoming clearer daily as [Russia] withdraws and regroups,” Melinda Simmons, the British ambassador to Ukraine wrote on Twitter. “Rape is a weapon of war. Though we don’t yet know the full extent of its use in Ukraine it’s already clear it was part of [Russia’s] arsenal.”

Artis Pabriks, Latvia’s defence minister, said: “After seeing Russian atrocities against civilians, if some in the free west still do not understand that we must do everything to make Russians lose and depart from [the] whole [of] Ukraine, then I am sorry for the west.”

Associated Press journalists in Bucha quoted local residents as saying the dead were civilians who had been killed by Russian soldiers without provocation. There was no immediate response from the Kremlin or Russian defence ministry.

Agence France-Presse reporters in the city said they saw at least 20 bodies on a single street.

Anatoly Fedoruk, mayor of Bucha, told AFP that 280 other bodies had already been buried in mass graves in the town.

On Saturday, the former chief prosecutor of UN war crimes tribunals Carla Del Ponte called for an international arrest warrant for Vladimir Putin, telling Swiss media that the Russian president was a “war criminal”.

“I hoped never to see mass graves again,” she told the Blick newspaper. “These dead people have loved ones who don’t even know what’s become of them. That is unacceptable.”