U.S. President Joe Biden called his Russian counterpart Vladimir Putin a war criminal on Wednesday amid the escalating brutality Ukraine while Ukrainian President Volodymyr Zelenskyy asked for more help from U.S. Congress.

"He’s a war criminal," Biden said of Putin as he left an unrelated event. It’s the sharpest condemnation yet of Putin and Russian actions by a U.S. official since the invasion of Ukraine. In a statement, Kremlin called Biden's remarks "unacceptable and unforgivable."

While other world leaders have used the words, the White House had been hesitant to declare Putin’s actions those of a war criminal, saying it was a legal term that required research.

But in a speech Wednesday, Biden said Russian troops had bombed hospitals and held doctors hostage. He approved an additional $800 million in security assistance to Ukraine, assuring the ally of "unprecedented" U.S. support in its war with Russia.

Addressing his Ukrainian counterpart Volodymyr Zelenskyy's remarks in his speech, Biden said the U.S. has offered Ukraine $1 billion in security aid, adding the U.S. will continue to give Ukraine weapons to fight and defend itself, offering humanitarian relief and support Ukraine's economy with additional financial assistance.

As expected, Biden gave no ground on the notion of a Western-imposed no-fly zone over the ex-Soviet state, which NATO has consistently rejected for fear of an escalation that could bring the alliance into direct conflict with Russian forces.

In his address to Congress, Zelenskyy compared the attacks in Ukraine to the Japanese attack on Pearl Harbor that pulled the U.S. to World War II, and begged lawmakers, and Biden directly, for more help.

"This is a terror that Europe has not seen for 80 years, and we are asking for our life, for an answer to this terror from the whole world. Is this a lot to ask for? To create a no-fly zone over Ukraine to save people?" Zelenskyy asked through an interpreter.

"If this is too much to ask, we offer an alternative," he said. "You know what kind of defense systems we need," adding that he knows the U.S. has them.

Biden has referred to the creation of a no-fly zone over Ukraine as "World War III," and the Pentagon has refused requests to send fighter jets to Ukraine.