The United States is supporting a multinational team of independent war crimes investigators, including American experts, that are working with Ukraine's prosecutor-general on a probe of alleged Russian war crimes, U.S. State Department spokesperson Ned Price said on Monday.

The team is working in Eastern Europe, but not in Ukraine itself, Price said. He said the team is collecting, preserving and analyzing evidence with a view toward prosecutions and other forms of holding Russia accountable, Price said.

The United States is also supporting the effort through funding for non-government organizations that are part of the effort, Price said. He said he could not provide further details, including how many Americans are involved or how much funding is being allocated.

At the start of his briefing on Monday, Price noted the horrific reports from Bucha, Ukraine, and other towns outside of the capital of Kyiv, describing reports of "civilians, many with their hands tied, apparently executed in the streets, others in mass graves."

"We are seeing credible reports of torture, rape and civilians executed alongside their families," Price said. "There are reports and images of a nightmare litany of atrocities, including reports of land mines and booby traps left behind by Putin's forces to injure even more civilians and slow the stabilization and recovery of devastated communities after they failed in their objective and withdrew."

U.S. Secretary of State Antony Blinken will discuss the reported atrocities with his NATO foreign minister counterparts during the Western military alliance's spring meeting this week, Price said.

President Joe Biden said earlier Monday that he is calling for more sanctions to be imposed on Russia in light of the reports from Bucha.

"We're continuously tightening sanctions and preparing for additional sanctions, jointly with our allies and partners," Price said.

-ABC News' Conor Finnegan

More than 1,500 residents fled Mariupol in private vehicles on, according to Ukrainian officials.

The mayor of Mariupol confirmed on Monday that a convoy of seven buses escorted by the International Committee of the Red Cross could not make it into his besieged city to evacuate residents. The mayor said residents were still able to flee the city using a single humanitarian corridor meant for private cars.

The mayor also said humanitarian cargo also failed to make it to Mariupol.

Ukrainian Deputy Prime Minister Iryna Vereshchuk said a total of 3,376 civilians evacuated the country on Monday, including 2,405 citizens of Zaporizhzhia in southeastern Ukraine who made it out via private transport.

France's foreign ministry announced that it decided Monday evening to expel "many" Russian diplomats.

France's decision came hours after the German foreign minister said her country is moving to expel a "significant" number of Russian diplomats following reports that Russian troops killed more than 400 civilians in the Ukraine city of Bucha.

A spokesperson for the French foreign ministry said the action by France is "part of a European approach" and expects other European Union nations to make similar announcements. The spokesperson did not disclose how many Russian diplomats will be expelled.

"France decided this evening to expel many Russian personnel with diplomatic status assigned to France whose activities are contrary to our security interests," the spokesperson said. "Our first responsibility is always to ensure the safety of French people and Europeans."

-ABC News' Christine Theodorou

About two-thirds of the Russian forces that were arrayed against the capital of Ukraine have withdrawn toward Belarus, according to a senior U.S. defense official.

The number of Russian forces being pulled back from Kyiv is up from an estimated 20% late last week, the official said.

Russian Deputy Defense Minister Col. Gen. Alexander Fomin told reporters last week that Russia's military activity was being dramatically curtailed near Kyiv and in Chernihiv in northern Ukraine in an attempt to increase "mutual trust and create conditions required" for further peace talks with Ukrainian negotiators.

The United States has been skeptical of Russia's promise to scale back its military activity near Kyiv.

Before repositioning its forces, Russia had close to 20 battalion tactical groups (BTGs) bearing down on Kyiv from the north and northwest, with each group comprised of 700 to 900 troops. Roughly 13 of those BTGs are now either in Belarus or on their way there.

The Pentagon believes the withdrawing forces will be resupplied and possibly reinforced in the north before heading back into Ukraine to fight elsewhere, the official said.

"Our best assessment –- and it is only an assessment -– is that they will be applied in the eastern part of the country in the Donbas region," the official said of the two separatist areas, Donetsk and Luhansk, in eastern Ukraine that Russian President Vladimir Putin recognized as independent self-proclaimed people's republics prior to the Feb. 24 invasion of Ukraine.

The United States has also seen some Russian troops leave the city of Sumy in northeast Ukraine and head north to the Russian border, according to the official.

Despite these movements, the official said the "vast majority" of the more than 125 BTGs that Russia committed to the invasion remain inside Ukraine.

-ABC News' Matt Seyler