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EXPLAINER-How could Russia's Putin be prosecuted for war crimes in Ukraine?

(Adds statement from Russia)

By Jacqueline Thomsen and Mike Scarcella

April 3 (Reuters) - Ukraine has accused Russia of war crimes in the town of Bucha just outside Kyiv, with Germany, France and others expressing outrage at the images of dead civilians.

The mayor of Bucha said on Saturday that 300 residents had been killed by Russian troops during a month-long occupation. Victims were seen by Reuters in a mass grave and lying in the streets.

Russia's defence ministry denied the Ukrainian allegations, saying footage and photographs showing bodies in Bucha were "yet another provocation" by the Ukrainian government.

Russia has previously denied targeting civilians and rejected allegations of war crimes in what it calls a "special military operation" in Ukraine.

Even before Bucha, Ukraine and its Western allies accused Russian forces of targeting civilians indiscriminately, citing the bombing in the southern port of Mariupol of a maternity hospital and a theater marked as sheltering children.

Legal experts said a prosecution of President Vladimir Putin or other Russian leaders would face high hurdles and could take years, as outlined below:

HOW IS A WAR CRIME DEFINED?

The International Criminal Court in The Hague defines war crimes as "grave breaches" of the post-World War Two Geneva Conventions, agreements which lay out the international humanitarian laws to be followed in war time. Breaches include deliberately targeting civilians and attacking legitimate military targets where civilian casualties would be “excessive,” legal experts said.

The USSR ratified the Geneva Convention in 1954. Russia in 2019 revoked its recognition of one of the protocols, but remains a signatory to the rest of the agreements. HOW MIGHT A CASE PROCEED?

The International Criminal Court's chief prosecutor, Karim Khan, said last month he had opened an investigation into possible war crimes in Ukraine.

Neither Russia nor Ukraine is a member of the ICC and Moscow does not recognize the tribunal. But Ukraine has given its approval to examine alleged atrocities on its territory dating back to Russia's annexation of Crimea in 2014.

Russia may decide not to cooperate with the ICC and any trial would be delayed until a defendant is arrested.

WHAT IS THE STANDARD OF PROOF?

The ICC will issue an arrest warrant if prosecutors can show "reasonable grounds to believe" war crimes were committed. To obtain a conviction, the prosecutor would have to prove a defendant's guilt beyond a reasonable doubt, experts said.