(CNN) — President Joe Biden called Russian President Vladimir Putin a “war criminal” Wednesday, a rhetorical leap that came as civilian deaths mount in Ukraine.

It was the harshest condemnation of Putin’s actions from any US official since the war in Ukraine began three weeks ago. Previously, Biden had stopped short of labeling atrocities being documented on the ground in Ukraine as “war crimes,” citing ongoing international and US investigations.

But on Wednesday, speaking with reporters at an unrelated event, Biden affixed the designation on the Russian leader.

“I think he is a war criminal,” the President said after remarks at the White House.

The shift from the administration’s previous stance came after an emotional address to Congress from Ukrainian President Volodymyr Zelensky, who aired a video showing Ukrainians suffering amid Russia’s onslaught. Zelensky asked American lawmakers and Biden for more help defending itself, including a no-fly zone and fighter jets.

Biden responded in his own address a few hours later, laying out new American military assistance to Ukraine — including anti-aircraft and anti-armor systems, weapons and drones — but stopping short of acceding to Zelensky’s requests.

Still, Biden acknowledged the horrors transpiring on the ground.

“We saw reports that Russian forces were holding hundreds of doctors and patients hostage in the largest hospital in Mariupol,” Biden said. “These are atrocities. They’re an outrage to the world. And the world is united in our support for Ukraine and our determination to make Putin pay a very heavy price.”

It wasn’t until a few hours after that that Biden responded to a question about Putin being a war criminal. Biden initially said “no,” but immediately returned to a group of reporters to clarify what had been asked. When asked again whether Putin was a war criminal, he answered in the affirmative.

On Thursday, Kremlin spokesman Dmitry Peskov told reporters Biden’s remarks were “absolutely unacceptable and inexcusable.”

Officials, including Biden, had previously avoided saying war crimes were being committed in Ukraine, citing ongoing investigations into whether that term could be used. Other world leaders have not been as circumspect, including British Prime Minister Boris Johnson, who said last week war crimes were being committed. The International Criminal Court at the Hague has also opened an investigation into war crimes. And the US Senate unanimously asked for an international investigation into war crimes on Tuesday. US Ambassador to the United Nations Linda Thomas-Greenfield said last week that actions committed by Russia against the Ukrainian people “constitute war crimes,” marking the first time a senior US official directly accused Moscow of war crimes since last month’s attack on Ukraine began.

In Poland last week, Vice President Kamala Harris called for international investigations into war crimes, and made clear she believed atrocities were underway. She said the intentional targeting of civilians would constitute war crimes.

After Biden delivered his assessment, the White House said the administration’s investigation into war crimes would continue.

“The President’s remarks speak for themselves,” press secretary Jen Psaki said. She said Biden was “speaking from the heart.”

State Department spokesman Ned Price echoed Psaki later Wednesday, telling CNN’s Erin Burnett on “OutFront” that “when you are speaking from the heart, speaking as a human and you’re seeing what we’ve all seen, these searing images on TV, a Russian strike on a maternity hospital in Mariupol, strikes against residential buildings, against schools, against civilian neighborhoods, it’s hard not to walk away with that conclusion.”

“What we are doing here at the State Department, we are collecting every single piece of information, we’re evaluating it, we’re documenting it and sharing it with our partners. There is a process that is involved in this and there are people working almost around the clock to document, evaluate, share as we all watch what is happening with some horror.”

Pressed as to how Putin’s actions don’t currently amount to war crimes, Price reiterated that “there is a formal process here at the department under international humanitarian law to document war crimes. We’re involved in that.”

While the term “war crimes” is often used colloquially — as Biden appeared to be doing Wednesday — they do have a legal definition that could be used in potential prosecution. That includes in the Geneva Convention, which specifies intentional targeting of civilians as a war crime.

Yet in order to prosecute a war crime, solid evidence is required. And for Russian officials to be held accountable, they would need to travel outside of the country.

Still, an official designation of war crimes — backed up with evidence — would still present the West with a symbolic tool in framing Putin’s actions in Ukraine.

Biden has come under increasing pressure to do more to help besieged Ukrainians as Russia’s campaign intensifies. On Wednesday, a theater in Mariupol where civilians were sheltering was bombed, the latest example of Russia’s indiscriminate shelling.

The pressure was only likely to increase after Zelensky’s dramatic appeal to lawmakers for more help. He compared what is happening in Ukraine to Pearl Harbor and September 11, and said “we need you right now” to offer more support.

Biden watched the address from the library of his private residence, and later called it a “convincing” and “significant” speech.

“Putin is inflicting appalling, appalling devastation and horror on Ukraine, bombing apartment buildings, maternity wards, hospitals,” he said afterward. “I mean, it’s godawful.”

Next week, Biden plans to travel to Brussels for an extraordinary session of NATO leaders, where he hopes to demonstrate western unity amid Russia’s aggression.

This story has been updated with additional reporting.

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