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Ukraine and Western nations on Sunday accused Russian troops of war crimes after the discovery of mass graves and "executed" civilians near Kyiv, prompting vows of action at the International Criminal Court.

Britain, France, Germany, the US and NATO all voiced horror at Ukrainian reports on Saturday of nearly 300 bodies lying in the street in Bucha, with some appearing to have been bound by their hands and feet before being shot.

City mayor Anatoly Fedoruk told AFP that 280 other bodies had been buried in mass graves. One rescue official said 57 people were found in one hastily dug trench behind a church. 

About 10 were either unburied or only partially covered by earth.

Ukraine's Foreign Minister Dmytro Kuleba called it a "deliberate massacre" while President Volodymyr Zelensky said Russian forces were committing "genocide".

His spokesman, Sergiy Nikiforov, said earlier the Bucha killing "looks exactly like war crimes".

"We found mass graves. We found people with their hands and with their legs tied up... and with shots, bullet holes, in the back of their head," he told the BBC. 

"They were clearly civilians and they were executed."

US Secretary of State Antony Blinken called the killings "a punch to the gut" while NATO chief Jens Stoltenberg said the violence, unseen in Europe for decades, was "horrific" and "absolutely unacceptable".

Britain, France, Germany, Italy and the European Union all called for those responsible to be brought to book at the international tribunal in The Hague.

Ukraine's Kuleba called for G7 nations to impose immediate "devastating" sanctions against Russia as a result. 

But despite Western action targeting oligarchs and businesses - and calls to go further - the Kremlin said it was not possible to isolate Russia entirely.

"There can be no complete vacuum or isolation of Russia. It is technologically impossible in the modern world," Kremlin spokesman Dmitry Peskov told Russian state television.

The world is "much larger than Europe", he said, adding: "Sooner or later we will have to build a dialogue, whether some overseas want it or not."

'Tormented Ukraine'

Europe's worst conflict in decades, sparked by Russia's invasion on February 24, has already left some 20,000 people dead, according to Ukrainian estimates.

Nearly 4.2 million Ukrainians have fled the country, with almost 40,000 pouring into neighbouring countries in the last 24 hours alone, the UN refugee agency said.

Nearly 6.48 million were estimated to be displaced inside Ukraine, the International Organisation for Migration has assessed. 

Pope Francis, on a visit to Malta on Sunday, made a plea for refugees fleeing the "sacrilegious war" in "tormented Ukraine" to be welcomed.

Several Western leaders, including US President Joe Biden, have already accused Russia's Vladimir Putin of being a "war criminal".

Human Rights Watch said Russian troops may have committed possible war crimes against civilians in occupied areas of Chernigiv, Kharkiv and Kyiv, including rape and summary execution.

Ukraine's President Volodymyr Zelensky has also alleged Russian soldiers planted mines and other booby traps as they withdraw from northern Ukraine, warning returning residents to be wary of tripwires and other dangers.

Odessa hit

The war crimes claims came as the Black Sea port city of Odessa, which has largely been spared in the conflict, was hit by air strikes apparently targeting key infrastructure.

Plumes of thick black smoke billowed over the strategic port city, after a series of blasts shook residents awake at about 6:00 am (0300 GMT).

"We were woken up by the first explosion then we saw a flash in the sky, then another, then another. I lost count," one local man, Mykola, 22, told AFP.

Russia's defence ministry said it had targeted an oil refinery and three fuel storage facilities with "high-precision sea and air-based missiles".

The depots were supplying fuel to Ukrainian troops, it added.

Anton Herashchenko, an adviser to Ukraine's interior minister, said: "Some of the missiles were shot down by air defence."

The strikes came as top UN humanitarian envoy Martin Griffiths was expected in Moscow then Kyiv to seek a halt to the fighting. 

Peace talks 

On peace talks, Russia's chief negotiator Vladimir Medinsky said it was too early for a top-level meeting between Zelensky and Putin on ending the conflict.

He said Kyiv had become "more realistic" in its approach to issues related to the neutral and non-nuclear status of Ukraine but a draft agreement for submission to a summit meeting was not yet ready.

And he said he did not share the "optimism" of Ukraine's negotiators on the possibility of talks between the two countries' leaders in Turkey.

His Ukrainian counterpart, David Arakhamia, had said on Saturday that Moscow had "verbally" agreed to key Ukrainian proposals, raising hopes that talks to end fighting were moving forward.

Ukraine has proposed abandoning its aspirations to join NATO and declaring official neutrality, if it obtains security guarantees from Western countries. It would also pledge not to host any foreign military bases.

It has proposed to temporarily put aside the question of Crimea, annexed by Russia in 2014, and two breakaway territories in the eastern Donbas region that Russia has recognised as independent.

Medinsky said Russia's position on Crimea and the Donbas "remains unchanged" and that talks would resume by video conference on Monday.

'Liberated'

As Russian forces withdraw from some northern areas, Moscow appears to be focusing on eastern and southern Ukraine, where it already holds swathes of territory.

UK Defence Intelligence said early Sunday that Russian air activity in the last week had been concentrating on southeastern Ukraine, "likely as a result of Russia focusing its military operations in this area".

But it said Russia was struggling to find and destroy air systems, which has "significantly affected their ability to support the advance of their ground forces".

In his latest video message, Zelensky said Russian troops wanted to seize the disputed Donbas region and southern Ukraine, promising "to defend our freedom, our land and our people".

Ukraine on Saturday claimed progress against Russian forces, saying Irpin, Bucha, Gostomel and the whole Kyiv region had been "liberated".

NATO's Stoltenberg, however, cautioned that Russia's claim to be pulling troops away from Kyiv was "not a withdrawal" but Russia repositioning its troops.

Evacuation bid

Russia's efforts to consolidate its hold on southern and eastern areas of Ukraine have been hampered by the resistance of Mariupol despite devastating attacks lasting weeks.

At least 5,000 residents have been killed in the besieged southern port city, according to officials, while the estimated 160,000 who remain face shortages of food, water and electricity.

The International Committee of the Red Cross (ICRC) said its team left for Mariupol on Saturday to make another attempt at conducting an evacuation, after being forced to turn back the day before.

A Lithuanian filmmaker, Mantas Kvedaravicius, 45, was killed as he tried to flee, the Ukrainian military said.