The International Criminal Court's (ICC) top prosecutor called Ukraine "a crime scene" after visiting the town of Bucha near Kyiv, where Russian forces are accused of carrying out a massacre of civilians.

"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. We have to pierce the fog of war to get to the truth," Karim Khan told reporters.

U.S. President Joe Biden for the first time accused Russian President Vladimir Putin's forces of committing genocide in Ukraine on Tuesday.

"Yes, I called it genocide," Biden told reporters, hours after employing the term during a speech in Iowa – its first use by a member of his administration.

"We'll let the lawyers decide internationally whether or not it qualifies, but it sure seems that way to me," Biden said. "It's become clearer and clearer that Putin is just trying to wipe out the idea of even being able to be a Ukrainian."

While the toll on towns occupied during the monthlong offensive to take Kyiv like Bucha is still coming to light, the heaviest civilian toll is feared to be in Mariupol, where Ukrainian President Volodymyr Zelenskyy said he believed Russia had killed "tens of thousands".

Moscow is believed to be trying to connect occupied Crimea with Russian-backed separatist territories Donetsk and Luhansk in Donbass and has laid siege to the strategically located city.

Experts say its fall is inevitable, but as the fighting drags toward its seventh week, the Ukrainian army is still clinging on.