The International Criminal Court's chief prosecutor visited the town of Bucha on Wednesday -- the scene of hundreds of civilian killings which Ukraine has blamed on Russian forces who occupied it for several weeks.
"Ukraine is a crime scene. We're here because we have reasonable grounds to believe that crimes within the jurisdiction of the court are being committed," Karim Khan told reporters
"We have to pierce the fog of war to get to the truth. That requires independent, impartial investigation," he said.
Khan said an ICC forensic team was set to work "so that we can really make sure we separate truth from fiction".
"We have to keep an open mind and we have to follow the evidence," he said.
"The law needs to be mobilised and sent into battle to protect civilians".
Russia has denied responsibility for the deaths and President Vladimir Putin has dismissed reports of Russian soldiers shooting civilians as "fake".