Biden accuses Putin of genocide

Biden's accusation came as Moscow -- already accused by the West of widespread atrocities against civilians -- appears to be readying a massive offensive across Ukraine's eastern Donbas region that Washington warned might involve chemical weapons.

In Mariupol, where strikes continued to pummel the battered city, more than 1,000 Ukrainian soldiers have surrendered, Russia's defence ministry said Wednesday. Ukraine did not confirmed the claim.

Following its pullback from areas north of Ukraine's capital, Kyiv, Russia is refocusing its efforts eastward, the new frontline of the nearly seven-week war.

It appears aimed at capturing more territory in Donbas, where Russian-backed separatists control the Donetsk and Lugansk regions, to create a solid southern corridor to occupied Crimea.

Ukrainian authorities have been urging people to flee west in advance of the expected Russian offensive but on Wednesday, all humanitarian corridors were halted, Deputy Prime Minister Iryna Vereshchuk said.

"The situation along the routes is too dangerous," Vereshchuk said, claiming Russian forces around Zaporizhzhia in the south were blocking buses transporting the evacuated, while shooting at fleeing civilians in Lugansk.

In the past 24 hours, seven civilians were killed by Russian shelling in the northeastern Kharkiv region, said regional governor Oleg Synegubov on social media.

Biden's charge of genocide was the strongest accusation yet from Washington against Putin, yet one that Ukrainian President Volodymyr Zelensky has repeatedly levelled.

Biden previously called Putin a "war criminal" following the discovery of hundreds of civilians killed in Bucha and neighbouring Kyiv suburbs held by Russian forces that sparked global condemnation.

"Yes, I called it genocide," said Biden, defending his use of the term Tuesday during a speech, while saying he'd let lawyers decide "whether or not it qualifies" as such.

"It's become clearer and clearer that Putin is just trying to wipe out the idea of even being able to be a Ukrainian."

The International Criminal Court's chief prosecutor Karim Khan said Wednesday that "Ukraine is a crime scene", speaking on a visit to Bucha.

"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," he told reporters.

The worst civilian toll is feared to be in Mariupol, where Zelensky accuses Russia of killing "tens of thousands".

On Wednesday, the Ukrainian army said on Telegram that air strikes continued, particularly targeting its port and the huge Azovstal iron and steel works.

The maze-like complex has been a focus of resistance in Mariupol, with fighters using a tunnel system below the vast industrial site to slow Russian forces down.

"It's a city within a city," said Eduard Basurin, a representative for pro-Russian separatists in Donetsk, citing subterranean areas that cannot be bombed from above.

"You have to go underground to clean them out, and that will take time."

US Secretary of State Antony Blinken said he could not confirm allegations that Russia had used chemical weapons in the area, but Washington had "credible information" Russia might use tear gas mixed with chemical agents in the besieged port.