The besieged Ukrainian city of Mariupol seems to be facing particular brutalization by Russian forces, including "some of the most egregious accounts of what could constitute war crimes," U.S. Department of State spokesperson Ned Price said Tuesday.

The southeastern coastal city's strategic location could be a factor, Price told reporters during a briefing. There may also be a "particular vengeance against this population," he added, with Putin "having been under the impression -- whether he was misinformed or just unwitting of reality -- that his forces would not be greeted as anything other than the aggressors that they are."

"Every day when it comes to potential atrocities, we are seeing, we are all witnessing additional evidence that the Russians are in fact committing atrocities, committing acts that may go above that threshold to include potential war crimes," Price said.

Price said the U.S. still hasn't confirmed Russia's claims that it has evacuated thousands of Ukrainian civilians from the city to Russian territory.

-ABC News' Conor Finnegan

Russia's Defense Ministry claimed Tuesday that it has evacuated 366,182 people from Ukraine to Russia since the invasion began.

Russia said 19,442 people, including 3,448 children, were evacuated from the Luhansk and Donetsk regions to Russia on Monday, without the participation of Ukrainian authorities, Mikhail Mizintsev, the head of Russia's National Defense Control Center, claimed in a press briefing.

Mizintsev claimed more than 68,000 residents were also evacuated from Mariupol without the Ukrainian authorities' involvement.

He claimed these people "are now completely safe under the Russian Federation's protection, and all those in need are receiving necessary aid."

Russia claimed it opened humanitarian corridors from Kyiv, Chernihiv, Sumy and Kharkiv on Tuesday.

White House national security adviser Jake Sullivan said there are no signs that China has given any military assistance to Russia to help with the war in Ukraine.

President Joe Biden held a video call with Chinese President Xi Jinping on Friday and Sullivan met with his counterpart in Rome on Monday over U.S. concerns that Beijing would aid Moscow.

Over a week ago, the U.S. confirmed that Russia asked China for military support and other aid since the invasion began.

"What I can tell you is we have not seen … the provision of military equipment by China to Russia," Sullivan said. "The president made clear to President Xi the implications and consequences of any such provision of equipment, and they very well understand one another."

When pressed on whether NATO allies would "put specific concrete steps in place" if China does provide assistance, Sullivan said Biden would be consulting with NATO and the EU.

“On April 1, the European Union is having a summit with China. And so this will be an opportunity, Thursday, for the United States and our European partners, to coordinate closely on what our message is. We believe we're very much on the same page with our European partners, and we will be speaking with one voice on this issue," Sullivan said.

-ABC News' Elizabeth Schulze

National security adviser Jake Sullivan said President Joe Biden and NATO allies will roll out new coordinated plans on Thursday in response to Russia's continued invasion.

Sullivan said Biden is traveling to Europe "to ensure we stay united" with our allies and to send a message that they are committed to help Ukraine "for as long as it takes."

"There will be hard days ahead in Ukraine. Hardest for the Ukrainian troops on the front lines and the civilians under Russian bombardment. This war will not end easily or rapidly," Sullivan said.

Sullivan said NATO allies will announce new sanctions, tighten existing ones, adjust troops on the Eastern Flank and outline plans to reduce Europe’s dependence on Russian energy.

Sullivan did not disclose any specific details about the sanctions but hinted that new ones will focus on a joint effort to crack down on countries helping Russia "undermine, weaken or get around the sanctions," calling it an "important part" of this next phase.

"We have applied an enormous amount of economic pressure, and in order to sustain and escalate that pressure over time, part of that is about new designations, new targets, but a big part of it is about effective enforcement and evasion, applying the lessons that we've learned from other circumstances where we have, in fact, imposed sanctions on countries," Sullivan said.

In Poland, Sullivan said Biden will "engage" with U.S. troops, hold a bilateral meeting with President Duda and meet with experts involved in the humanitarian response.

-ABC News' Justin Gomez

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