President Joe Biden called Vladimir Putin a "war criminal" on Wednesday just hours after Ukrainian President Volodymyr Zelenskyy appealed to the U.S. Congress for more assistance in the fight against Russian forces invading his country. 

The president had initially said "no" when asked by Fox News reporter Jacqui Heinrich if he considers Putin a war criminal, but circled back about 30 seconds later and said, "Did you ask me whether I would call… Oh, I, I think he is a war criminal."

Earlier in the day, Biden condemned Russia's invasion and said the U.S. is sending more weapons to Ukraine, including anti-aircraft, anti-armor weapons, body armor, guns, and drones. 

"Putin is inflicting appalling, appalling devastation and horror on Ukraine, bombing apartment buildings, maternity wards, hospitals. I mean, it's God awful," Biden said. "The world is united in our support for Ukraine and our determination to make Putin pay a very heavy price."

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President Joe Biden and Russian President Vladimir Putin, arrive to meet at the 'Villa la Grange', in Geneva, Switzerland, June 16, 2021.  (AP Photo/Patrick Semansky, File)

White House Press Secretary Jen Psaki said at a briefing that the president's remarks "speak for themselves," but that a "legal process" is still underway at the State Department. 

"He was speaking from his heart and speaking from what we’ve seen on television, which is barbaric actions by a brutal dictator through his invasion of a foreign country," Psaki said. 

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A spokesperson for the Kremlin, Dmitry Peskov, responded by saying that Biden's comments were "unforgivable," according to the Russian media outlet TASS. 

"We consider unacceptable and unforgivable such rhetoric of the head of state, whose bombs killed hundreds of thousands of people around the world," Peskov said. 

Until Wednesday, Biden had resisted calling the Russian President a war criminal, even as multiple lawmakers called for Putin to be investigated for war crimes. 

On Tuesday evening, the U.S. Senate unanimously approved a measure to investigate Putin and other Russian officials for "war crimes" and "crimes against humanity." 

Russia's military attacked a theater in Mariupol on Wednesday where "hundreds of innocent civilians were hiding," Ukraine Foreign Minister Dmytro Kuleba said. 

"The building is now fully ruined. Russians could not have not known this was a civilian shelter," Kuleba wrote on Twitter. "Save Mariupol! Stop Russian war criminals!"

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Russian forces have also been accused of using cluster munitions and vacuum bombs in the war, which violate international law when used indiscriminately against civilians. 

About 700 civilians have been killed since Russia invaded Ukraine on Feb. 24, but the United Nations says the actual number is likely much higher. 

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Ukrainian President Volodymyr Zelenskyy delivers a virtual address to the U.S Congress at the Capitol, in Washington, U.S., March 16, 2022.  (Drew Angerer/Pool via REUTERS)

In a public address to Congress on Wednesday, Zelenskyy showed graphic videos of civilian casualties and devastation inflicted by Russian forces on his country. 

"We need you right now," Zelenskyy told the lawmakers. "I call on you to do more."

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The International Criminal Court said earlier this month that it is opening an investigation into potential war crimes by Putin in his invasion of Ukraine.