European Union leaders said Wednesday they were preparing a new round of economic sanctions against Russia, as outrage grew over civilian deaths in the Ukrainian town of Bucha.

"We have all seen the haunting images of Bucha. This is what is happening when Putin's soldiers occupy Ukrainian territory," European Commission President Ursula von der Leyen said. "They call this liberation. I call this war crimes. The Russian authorities will have to answer for them."

The sanctions to be proposed may include a ban on importing Russian coal, bans on transactions with four Russian banks, and a ban on Russian ships at EU ports, among other measures.

The fifth round of sanctions "will not be our last," von der Leyen said. U.S. officials are also expected to announce new sanctions on Wednesday, sources told ABC News.

Russian forces are continuing their airstrikes in the besieged Ukrainian port city of Mariupol, according to the U.K. Ministry of Defense.

"The humanitarian situation in the city is worsening," the ministry said Wednesday in an intelligence update. "Most of the 160,000 remaining residents have no light, communication, medicine, heat or water."

Russian troops have prevented humanitarian access to the southern city, a move the ministry said was as part of a strategy to pressure Ukraine to surrender.

In an interview with MSNBC on Tuesday night, U.S. ambassador to the United Nations Linda Thomas-Greenfield said the United States could not remove Russia from the U.N.'s most powerful body, the Security Council.

"They are a member of the Security Council. That's a fact. We can't change that fact, but we certainly can isolate them in the Security Council," Thomas-Greenfield told MSNBC.

That's separate from the push to remove Russia from the U.N. Human Rights Council, which Thomas-Greenfield told NPR they hope to bring to the U.N. General Assembly for a vote as soon as Thursday.

"I know we're going to get" the necessary two-thirds majority, she told CNN.

Thomas-Greenfield also described what it was like in the room on Tuesday as Ukrainian President Volodymyr Zelenskyy's graphic video finally played for the U.N. Security Council. She told MSNBC it was the first time she saw the uncensored video of the war's victims.

"We were all speechless. We had all seen various videos showing atrocities. But they all covered up the real, you know, the real people that were there - they were all blurred," Thomas-Greenfield said. "This was the first time I've seen that video without the bodies being blurred. And it was horrific. And there was silence in the room. I can tell you that people were horrified."

-ABC News' Conor Finnegan

The U.S. will be sending an additional $100 million in Javelin anti-tank missiles to Ukraine, a White House official confirmed to ABC News. The weapons will be coming from existing military stockpiles.

The White House later released a memorandum from President Joe Biden saying he would be using drawdown powers to release "an aggregate value of $100 million in defense articles and services of the Department of Defense, and military education and training, to provide assistance to Ukraine."

Pentagon officials have said anti-tank weapons provided by the U.S. and other partner countries have been very successful in staving off Russian troops and bogging down vehicle movement.

-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