The U.S. has assessed that "thousands" of Ukrainian civilians have been killed by the war, Ambassador-at-Large for Global Criminal Justice Beth Van Schaack said at a briefing on Wednesday.

Van Schaack declined to speak to particular incidents that back up this assessment, but when asked about the attack on the Mariupol theater, which had been marked that children were among those sheltering inside, she said it was a civilian target that the Russians should not have hit.

The U.S. earlier formally declared that Russian forces have committed war crimes in Ukraine.

-ABC News' Conor Finnegan

At least 977 civilians have been killed and 1,594 others have been injured in Ukraine since Russian forces invaded on Feb. 24, according to the latest figures from the Office of the United Nations High Commissioner for Human Rights (OHCHR).

"Most of the civilian casualties recorded were caused by the use of explosive weapons with a wide impact area, including shelling from heavy artillery and multiple-launch rocket systems, and missile and air strikes," the OHCHR said in a statement Wednesday.

The OHCHR noted that the actual number of casualties are believed to be "considerably higher," because the receipt of information from some areas with intense hostilities, like the southeastern port city of Mariupol, have been delayed and many reports are still pending corroboration.

Other areas where the number of casualties are still being corroborated include Volnovakha in the Donetsk Oblast, Izium in the Kharkiv Oblast, Sievierodonetsk and Rubizhne in the Luhansk Oblast, and Trostianets in the Sumy Oblast, where there are allegations of numerous civilian casualties, according to the OHCHR. Casualty numbers from these regions are not included.

-ABC News' Christine Theodorou

U.S. Secretary of State Antony Blinken on Wednesday announced that the State Deptartment has made a formal assessment that Russian forces have committed war crimes in Ukraine.

Last week, President Joe Biden said he believed Russia's President Vladimir Putin was a war criminal and Blinken told reporters he personally believed war crimes had been committed. But now, the agency has made a formal determination, Blinken said in a statement.

"Based on information currently available, the U.S. government assesses that members of Russia's forces have committed war crimes in Ukraine," Blinken said, citing a "careful review of available information from public and intelligence sources."

Blinken added: "As with any alleged crime, a court of law with jurisdiction over the crime is ultimately responsible for determining criminal guilt in specific cases. The U.S. government will continue to track reports of war crimes and will share information we gather with allies, partners, and international institutions and organizations, as appropriate."

Blinken said there are "numerous credible reports of indiscriminate attacks and attacks deliberately targeting civilians, as well as other atrocities."

"Russia’s forces have destroyed apartment buildings, schools, hospitals, critical infrastructure, civilian vehicles, shopping centers, and ambulances, leaving thousands of innocent civilians killed or wounded," Blinken said.

-ABC News' Conor Finnegan

In an address to French lawmakers on Wednesday, Ukraine's President Volodymyr Zelenskyy said Mariupol resembles the ruins of Verdun following the largest battle fought during WWI.

He said Russia brought "state terror" to Ukraine.

Zelenskyy called for increased sanctions against Russia, more arms for Ukraine and for French companies to leave the Russian market, naming Renault, Auchan and Leroy Merlin.

"You can help us. I know you can!" Zelenskyy said.

-ABC News' Christine Theodorou

All Russian troops have left the Ukrainian cities of Kyiv and Chernihiv, withdrawing north toward the borders of Belarus and Russia to consolidate before likely redeploying to the Donbas region in eastern Ukraine, a senior U.S. defense official told reporters Wednesday.

But even with the Russians gone, the territory remains treacherous.

"There are some indications that they left behind mines and things like that, so the Ukrainians are being somewhat careful in some areas north of Kyiv as they begin to clear the ground and clear the territory and re-occupy it," the official said.

While the U.S. hasn't yet seen these troops redeploy elsewhere in Ukraine, it'll likely happen soon, according to the official. Ukrainian forces are preparing for a major fight in Donbas, the official said.

The official also said the Pentagon is "monitoring" an apparent nitric acid explosion in Ukraine's Luhansk region, which Russia blamed on Ukraine.

"We've seen the Russians claim that this was a Ukrainian attack on this. We do not believe that is true," the official said. "We do believe that the Russians are responsible, but exactly what they used when they did it, why they did it, what the damage is, we just don't have that level of detail," the official said.

The official also noted that a small number of Ukrainians currently in the U.S. for "professional military education" were pulled aside for a couple days of training on Switchblade drones, which the U.S. is sending overseas as part of its military aid, according to the official.

"Although it's not a very difficult system to operate, we took advantage of having them in the country to give them some rudimentary training on that," the official said.

-ABC News' Matt Seyler