By Marko Djurica and Abdelaziz Boumzar

BUCHA, Ukraine (Reuters) - The United States and European countries pledged on Monday to punish Moscow over civilian killings in northern Ukraine, where a mass grave and tied bodies of people shot at close range were found in a town seized back from Russian forces.

The deaths in Bucha, outside Kyiv, drew pledges of further sanctions against Moscow from the West, possibly including some restrictions on the billions of dollars in energy that Europe still imports from Russia.

The discoveries came against a backdrop of artillery barrages in Ukraine's south and east, where Russia says it is now focusing after it fell short in attempts to take major cities in the heart of the country.

In a visit to Bucha, Ukrainian President Volodymyr Zelenskiy said "these are war crimes and will be recognised by the world as genocide."

Russia denied any accusations related to the murder of civilians, including in Bucha. Its envoy to the United Nations, Vasily Nebenzya, said Russia will present "empirical evidence" to the Security Council on Tuesday showing its forces were not involved in atrocities.

Zelenskiy said the killings had made it harder for Ukraine to negotiate with Russia. Neither side offered updates on peace talks that had been set to resume on Monday.

Taras Shapravskyi, Bucha's deputy mayor, said around 50 victims of extra-judicial killings by Russian troops had been found there after Kremlin forces withdrew late last week.

Reuters saw one man sprawled by the roadside, his hands bound behind his back and a bullet wound to his head. Hands and feet poked through red clay at a mass grave by a church where satellite images showed a 45-foot-long trench.

Ukraine's Foreign Minister Dmytro Kuleba said the evidence of civilian killings was just the "tip of the iceberg", with Ukrainian forces yet to reach all areas vacated by Russian troops, and showed the need for tougher sanctions on Moscow.

U.S. President Joe Biden called for a war crimes trial against Russia's President Vladimir Putin, calling him "brutal".

Makeshift Burials

Ukrainian authorities said they had found 421 civilian casualties near Kyiv by Sunday and were investigating possible war crimes in Bucha, a description also used by French President Emmanuel Macron and, in reference to Russia's broader offensive, by the U.N. High Commissioner for Human Rights.

Reuters saw more makeshift burials elsewhere but could not independently verify the number of dead or who was responsible.

In the village of Motyzhyn, west of Kyiv, Reuters reporters saw three bodies in a forest grave. An adviser to Ukraine's interior ministry said the victims were the village's leader, Olha Sukhenko, and her husband and son.

Ihor, a local resident who said he was a relative of the family and did not give his surname, told Reuters: "I don't know what they were killed for. They were peaceful, kind people."

Zelenskiy has used the term genocide at various times during the war, decrying what he calls an intent to eliminate the nation by Putin, who has questioned Ukraine's legitimate, independent history from Russia.

Washington, at Kyiv's request, was backing a multinational team of prosecutors who would help collect and analyse evidence of atrocities with a view towards pursuing accountability, the U.S. State Department said.

Russia rejects allegations of war crimes in what it calls a "special military operation" aimed at demilitarising and "denazifying" Ukraine. Ukraine says it was invaded without provocation.

German Chancellor Olaf Scholz said Putin and his supporters would "feel the consequences" of events in Bucha and that Western allies would agree further sanctions against Moscow in the coming days. The Biden administration said new U.S. sanctions would be announced this week.

German Defence Minister Christine Lambrecht said the European Union must discuss banning Russian gas, though other officials urged caution around measures that could touch off a European energy crisis.

Flattened By Shelling

On the other side of the country in Mariupol, a southeastern port that has been under siege for weeks, Reuters images showed three bodies in civilian clothes lying in the street, one against a wall sprayed with blood.

Outside a damaged apartment building, residents buried other dead in a shell crater. "It is easier to dig here," a resident said, saying four bodies were in the improvised grave.

Ukraine says it has evacuated thousands of civilians in the past few days from Mariupol, which is surrounded by areas held by Russian-backed separatists in the eastern Donbas region.

A team from the International Committee of the Red Cross was stopped during an attempt to reach Mariupol to evacuate civilians, and is now being held in a nearby town, a spokesperson said. Several previous ICRC attempts to reach the city were unsuccessful.

West of Mariupol, in the town of Mykolaiv, shelling killed ten people, including a child, and injured 46 others, regional administration head Oleksandr Senkevich said. Reuters was not immediately able to verify the report.

Russian Redeployments

Ukraine is preparing for what its general staff said were about 60,000 Russian reservists called in to reinforce Moscow's offensive in the east, after Russian forces became bogged down elsewhere in the face of unexpectedly lethal and mobile Ukrainian resistance using Western anti-tank weaponry.

Officials in Ukraine's northern regions, nearest Russia, said Russian troops there had fully withdrawn or significantly scaled back in number.

Over the weekend Ukraine said its forces had wrested back all areas around Kyiv, claiming complete control of the capital region for the first time since Russia launched its onslaught. It has since warned of attacks on Ukraine's second-largest city, Kharkiv, which abuts the Russian border.

A senior U.S. defence official said Russia had repositioned about two-thirds of its forces from around Kyiv, with many consolidating in close ally Belarus where they were expected to be resupplied.

Reuters could not independently confirm the statements about Russian troop movements.

Biden's national security adviser, Jake Sullivan, said Russia would likely to try to surround and overwhelm Ukrainian troops in eastern Ukraine.

"Russia could then use any tactical success it achieves to propagate a narrative of progress and mask... prior military failure," Sullivan said.