U.S. first lady Jill Biden will travel to Romania and Slovakia this week to meet with American soldiers, U.S. embassy staff as well as displaced Ukrainian families, the White House announced Monday.

Romania and Slovakia are hosting hundreds of thousands of refugees from Ukraine who were forced to flee their homes due to Russia's invasion.

According to a press release from the White House, Biden will depart the United States for Romania on Thursday evening. On Friday, she will visit Mihail Kogalniceau Airbase in southeastern Romania, where she will meet with U.S. military service members.

On Saturday, Biden will travel to Romania's capital, Bucharest, to meet with Romanian government officials, U.S. embassy personnel, humanitarian aid workers as well as educators who are helping teach displaced Ukrainian children. She will then travel to Slovakia's capital, Bratislava, to meet with U.S. embassy staff there, according to the White House.

On Sunday, which is celebrated as Mother's Day in the U.S., Biden will travel to the eastern Slovak city of Kosice and the small village of Vysne Nemecke, the largest of three border crossings between Slovakia and Ukraine, to meet with Ukrainian refugees, humanitarian aid workers as well as local Slovakians who are supporting the displaced families, according to the White House.

"On Mother's Day, she will meet with Ukrainian mothers and children who have been forced to flee their home country because of Putin's war," the White House said in a statement.

On Monday, Biden will meet with Slovakian government officials before heading back to the U.S.

-ABC News' Armando Garcia

A high-level U.S. congressional delegation led by House Speaker Nancy Pelosi met with Polish President Andrzej Duda in Warsaw on Monday, a day after meeting with Ukrainian President Volodymyr Zelenskyy in Kyiv.

"Our distinguished Congressional delegation came to Poland to send an unmistakable message to the world: that America stands firmly with our NATO allies in our support for Ukraine," Pelosi said in a statement.

Pelosi said their talks with Duda and other Polish officials in the Polish capital "will be focused on further strengthening our partnership, offering our gratitude for Poland's humanitarian leadership, and discussing how we can further work together to support Ukraine."

Earlier, Pelosi and the half dozen U.S. lawmakers with her traveled to the southeastern Polish city of Rzeszow, where they met with U.S. Army soldiers assigned to the 82nd Airborne Division stationed in Poland to reassure NATO allies and deter Russian aggression.

"These engagements are even more meaningful following our meeting in Kyiv with President Volodymr Zelenskyy and other top Ukrainian leaders," Pelosi said. "In that profound and solemn visit, our delegation conveyed our respect and gratitude to President Zelenskyy for his leadership and our admiration of the Ukrainian people for their courage in the fight against Russia's diabolical invasion. Our Members were proud to deliver the message that additional American support is on the way, as we work to transform President Biden’s strong funding request into a legislative package."

Pelosi, second in line to the U.S. presidency after the vice president, was the most senior American lawmaker to visit Ukraine since Russia launched its invasion on Feb. 24. The delegation's trip to the Ukrainian capital was not disclosed until they were safely out of the country.

-ABC News' Chad Murray

Russian forces resumed shelling the Azovstal steel plant in Mariupol on Sunday after some civilians inside the facility and in nearby homes were evacuated during a brief cease fire, according Ukrainian officials.

"They are shelling the plant with all kinds of weapons," said Denis Schlega, commander of the 12th Brigade of Operational Assignment in Mariupol.

Earlier Sunday, Ukrainian President Volodymyr Zelenskyy, the International Committee of the Red Cross, the United Nations and Russian Ministry of Defense all confirmed that some civilians were evacuated from the steel plant, where a Ukrainian military unit is making a last stand in the port city that is almost entirely under Russian control.

Zelenskyy said about 100 civilians were evacuated from the steel plant on Sunday and were being taken to Zaporizhia, a city under Ukrainian control.

The Mariupol City Council said in a statement that evacuations from Mariupol had stopped Sunday afternoon due to "security reasons." The city council said the evacuations would resume on Monday.

At least three civilians were killed and eight others injured on Sunday as a result of heavy shelling from Russian forces in the Kharkiv region in northeast Ukraine, according to a Ukrainian official.

The casualties were reported in the residential areas of Saltivka, Bohodukhiv and Zolochif, according to Oleg Sinegubov, head of the Kharkiv Regional Military Administration.

-ABC News' Christine Theodorou

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan