The International Committee of the Red Cross said Monday that its staff has been blocked again from entering Mariupol after failing to reach the besieged Ukrainian port city for several days.

An ICRC team tasked with escorting evacuation buses to and from Mariupol "was stopped on Monday while carrying out humanitarian efforts to help lead a safe passage corridor for civilians" and is now "being held in the town of Manhush," about 12 miles west of Mariupol, according to an ICRC spokesperson.

"The ICRC has been in direct contact with our colleagues and is speaking with the parties on all sides to bring clarity to the situation and allow them to resume their humanitarian work," the spokesperson told ABC News in a statement.

-ABC News' Brian Hartman

U.S. President Joe Biden said Monday that he is seeking more sanctions against Russia after horrific images surfaced of dead civilians lying in the bombed-out streets of Bucha, Ukraine.

Upon his return to the White House from Fort McNair, Biden said he will call for more sanctions but stopped short of accusing Russia of committing genocide in Ukraine.

"I'm seeking more sanctions, yes," Biden said, declining to offer specifics when pressed.

Biden described the situation in Bucha as "outrageous" and called Russian President Vladimir Putin "brutal."

"You may remember I got criticized for calling Putin a war criminal. Well, the truth of the matter -- you saw what happened in Bucha," Biden said. "This warrants he is a war criminal. But we have to gather the information. We have to continue to provide Ukraine with weapons they need to continue the fight. And we have to gather all the details. So this could be an actual -- have a war-crime trial. This guy is brutal. And what's happening with Bucha is outrageous. And everyone sees it."

Ukrainian officials accused Russian troops of committing genocide in Bucha, which was retaken by Ukrainian forces in recent days. Ukrainian leaders said 410 civilians were killed in the fighting in Bucha and that many were found with their hands bound and shot at close range.

Asked if he thought Russia was committing genocide, Biden replied, "No, I think it is a war crime."

-ABC News' Molly Nagle

The General Staff of the Ukrainian Armed Forces said Monday it is monitoring large movements of Russian troops and reinforcements in eastern Ukraine.

The General Staff said it expects Russian forces to launch a possibly major offensive in the Donbas region within the next 24 hours, particularly against the city of Severodonetsk, which is the administrative center of the government-controlled areas of the Luhansk Oblast.

Meanwhile, Donetsk Oblast Gov. Pavlo Kyrylenko has urged civilians to evacuate now, even from areas not close to the front lines.

Prior to Russia's invasion of Ukraine on Feb. 24, Russian President Vladimir Putin announced his recognition of two breakaway areas of eastern Ukraine's Donbas region that share a border with Russia -- the self-declared Donetsk and Luhansk people's republics. Russia-backed separatist forces have controlled these parts of Ukraine's Donetsk and Luhanks oblasts since 2014.

-ABC News' Yulia Drozd

Ukrainian President Volodymyr Zelenskyy visited on Monday the decimated city of Bucha, where Ukrainian officials have accused Russian troops of committing war crimes against civilians.

Zelenskyy toured the Kyiv suburb that was retaken by Ukrainian forces in recent days. Zelenskyy went to a road in the city littered with destroyed Russian equipment and he spoke to local residents.

Zelenskyy repeated accusations that Russia committed war crimes and genocide after Ukrainian officials said 410 people believed to have been civilians were found dead, many with their hands bound behind their backs and shot at close range.

Russian officials have denied the accusation and have requested the U.N. Security Council investigate.

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