US President Joe Biden has denounced Russian President Vladimir Putin, calling him "a war criminal" for the first time.

Biden made the comment on Wednesday in response to a reporter's question.

Russia's state-run Tass news agency quoted Kremlin spokesperson Dmitry Peskov as saying the comment was "unacceptable and unforgivable rhetoric."

White House Press Secretary Jen Psaki said the president was "speaking from his heart" and not making an official declaration. She added that the State Department is looking into whether Putin has committed war crimes.

Meanwhile, Biden responded to an address by Ukrainian President Volodymyr Zelenskyy with what Biden called "unprecedented assistance."

He said, "It's about freedom. It's about the right of people to determine their own future. It's about making sure Ukraine ... will never be a victory for Putin, no matter what advances he makes on the battlefield."

A defense package Biden pledged to give Ukraine includes anti-aircraft systems, drones and grenade launchers. Biden says US assistance this week alone adds up to one billion dollars.

He also promised more humanitarian aid, as millions of Ukrainians have flooded across borders into Poland and Moldova. The Biden administration plans to send food, medicine and other supplies.