First lady Jill Biden, in Eastern Europe to show American support for its NATO allies, made a surprise visit to Ukraine Sunday, where she met with that nation's first lady.

Biden's motorcade crossed into Ukraine from Slovakia, where earlier in the day she toured refugee aid stations.

In Ukraine, she visited a public school in the city of Uzhhorod, where she met privately with the first lady of Ukraine, Olena Zelenska, for about half an hour.

"I thought it was important to show the Ukrainian people that this war has to stop," Biden told reporters following the visit. "And this war has been brutal. And that the people of the United States stand with the people of Ukraine."

"We understand what it takes for the U.S. first lady to come here during a war where the military actions are taking place every day, where the air raid sirens are happening every day, even today," Zelenska said through a translator. "We all feel your support. We all feel the leadership of the U.S. president."

"We would like to note that Mother’s Day is a very symbolic day for us," Zelenska added. "Because we also feel your love and support during such an important day."

-ABC News' Armando Garcia

Emergency responders were searching on Sunday for dozens of missing civilians after Russian forces struck a school building being used as a shelter in Bilohorivka, a village in eastern Ukraine, a local official said.

About 90 civilians had taken shelter in the building before it was destroyed on Saturday, Serhiy Haidai, regional governor of Luhansk in eastern Ukraine, said.

About 30 people had been rescued and two bodies were found in the rubble, the local emergency services said. Rescue efforts resumed at dawn on Sunday, Haidai said on social media.

Russia will hold its annual Victory Day on Monday, a national holiday under Russian President Vladimir Putin that marks the Soviet victory over Nazi Germany in World War II.

From the start, the Kremlin had hoped to use Victory Day to celebrate its own triumph in Ukraine, but Russia’s offensive to take Donbas has badly stalled. Instead, the Kremlin is expected to use the day to try to galvanize support among Russians for the war.

The day is being watched closely because how Putin sells the war could give a sense of how long the Kremlin wants to fight.

Ukraine, Western countries and many experts fear Putin might use Victory Day to declare a national mobilization -- without which many military analysts doubt Russia can achieve even its already curtailed goals in Ukraine.

If Putin does not use Victory Day to declare a general mobilization, that could suggest the Kremlin doesn’t believe it has the political strength at home right now to do so.

-ABC News' Patrick Reevell

Russia’s offensive in eastern Ukraine could be just as dangerous as its failed first offensive north of Kyiv, CIA Director William Burns said Saturday.

Russian President Vladimir Putin is "in a frame of mind that he thinks he cannot afford to lose, so the stakes are quite high in this phase," Burns said during an event in Washington, D.C., hosted by the Financial Times.

Burns said he does not think that Putin has been deterred by the huge amount of Western military support for Ukraine and the resolve of Ukraine’s population and its military.

"I don't think this means that, you know, Putin is deterred at this point, because he staked so much on the choice that he made to launch this invasion that I think he's convinced right now that doubling down still will enable him to make progress," he said.

Burns called Putin's invasion a "profound mistake."

"It was Putin's biggest mistake in planning for this invasion," he said. "And then in launching it, don't underestimate Ukrainians. And I think it's equally a mistake for any of us to underestimate what they bring to the table in intelligence terms in defending their own country."

-ABC News' Luis Martinez

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