The justice chiefs of the members of the "Five Eyes" intelligence alliance gave strong support Thursday to Ukraine's efforts to prosecute war crimes arising from Russia's invasion.

The attorneys general of the United Kingdom, the United States, Australia, Canada, and New Zealand said they fully backed Ukrainian Prosecutor General Iryna Venediktova in ensuring accountability for war crimes committed since Russia attacked the country on February 24.

Ukraine authorities say they have opened thousands of cases into alleged crimes committed by Moscow's forces.

"We support the pursuit of justice by Ukraine and through other international investigations, including at the International Criminal Court" and other bodies," they said in a statement.

"We join in condemning the Russian Government for its actions, and call upon it to cease all violations of international law, to halt its illegal invasion and to cooperate in efforts to achieve accountability," they said.

Ivan Matysov (R) testifies in the case of fellow Russian soldier Vadim Shishimarin, the first Russian to stand trial on war crimes charges in Ukraine since Russia invaded the country
Ivan Matysov (R) testifies in the case of fellow Russian soldier Vadim Shishimarin, the first Russian to stand trial on war crimes charges in Ukraine since Russia invaded the country AFP / Sergei SUPINSKY

The statement came out one day after a Russian soldier pleaded guilty to murdering an unarmed 62-year-old man four days into the invasion, in Ukraine's first prosecution arising from the war.

Vadim Shishimarin, 21, asked for forgiveness in a hearing Thursday as prosecutors sought a life sentence.

"I know that you will not be able to forgive me, but nevertheless I ask you for forgiveness," he said, addressing the court and the wife of the man he killed.

"By this first trial, we are sending a clear signal that every perpetrator, every person who ordered or assisted in the commission of crimes in Ukraine shall not avoid responsibility," Venediktova said.

Trials began Thursday in Ukraine for two other Russian soldiers.