Russia-Ukraine updates: US sanctions Russian military shipbuilder, diamond miner

U.S. President Joe Biden will meet with Polish President Andrzej Duda as well as refugees from Ukraine in Warsaw on Saturday, according to White House national security adviser Jake Sullivan.

Speaking to reporters aboard Air Force One en route to Poland on Friday, Sullivan noted that Biden will also deliver a "major address" before departing Saturday.

"He will give a major address tomorrow that will speak to the stakes of this moment, the urgency of the challenge that lies ahead, what the conflict in Ukraine means for the world, and why it is so important that the free world sustain unity and resolve in the face of Russian aggression," Sullivan said. "He'll also talk about the context and history of this conflict and where he sees it going from here."

Upon arrival in Rzeszow, Poland, on Friday, Biden will meet with humanitarian aid groups as well as service members of the U.S. Army's 82nd Airborne Division, according to Sullivan.

"He will be able to talk through with a range of different humanitarian leaders and experts, both from the region and from the international community as well as the US government experts who are playing a key role in this, on how the efforts are going so far and what further steps need to be taken to make sure that we're investing those dollars as wisely as possible," Sullivan said. "He will also have the chance to visit with troops from the 82nd Airborne Division, who have been deployed to Poland to reassure our NATO ally and to deter further aggression on the eastern flank. And he will also get a briefing from the commanders of those units who will have the chance to lay out for him the various tasks and missions that the American troops stationed at the airfield here have been undertaking and continue to undertake."

-ABC News' Molly Nagle

Video has emerged showing Ukrainian rescuers working to remove unexploded devices from civilian homes amid the Russian invasion.

The video, released Friday by the State Emergency Service of Ukraine and verified by ABC News, shows pyrotechnic units in the northern city of Chernihiv using special equipment to carefully search for and remove shells, missiles and mines that landed in houses but didn't explode.

The State Emergency Service of Ukraine said it was called in to seize ammunition 18 times over the past day. The agency warned people not to approach the objects because they could explode "at any time" and to immediately report such findings to rescuers or police.

-ABC News' Victoria Beaule

U.S. President Joe Biden departed Belgium on Friday morning and was en route to Poland for the final leg of his four-day trip aimed at maintaining unity among allies and supporting Ukraine's defense against Russia.

Biden was seen boarding Air Force One in the European Union's de facto capital, Brussels, at 6:42 a.m. ET. He is expected to land in Rzeszow, Poland, at around 9:15 a.m. ET, where he will receive a briefing on the humanitarian response to the millions of people fleeing Ukraine in the wake of Russia's invasion. He will also meet with service members from the U.S. Army's 82nd Airborne Division.

-ABC News' Molly Nagle

About 300 people were killed last week in a Russian airstrike on a drama theater-turned-bomb shelter in the besieged Ukrainian city of Mariupol, the city's government said Friday, citing eyewitnesses.

"We didn't want to believe in this horror," the Mariupol City Council. said in a statement. "But the words of those who were inside the building at the moment of this terrorist act say the opposite."

As many as 1,500 civilians had been taking refuge in the grand, columned Donetsk Regional Theatre of Drama in central Mariupol when it was struck on March 16, according to the Ukrainian government. Satellite images showed huge white letters on the pavement in front of and behind the building spelling out "CHILDREN" in Russian -- "DETI" -- to alert warplanes to those inside.

Video circulating online and verified by ABC News shows the immediate aftermath of the strike on the theater. People covered in dust are seen trying to make their way out of the theater, walking down from the first floor staircase in an area of the building that was still standing at the time.

Since invading Ukraine on Feb. 24, Russian forces have been relentlessly bombarding Mariupol, destroying homes and leaving thousands of residents trapped. Ukraine has defied Russia's ultimatum for its troops to lay down arms and surrender the strategic southeastern port city of 430,000.

-ABC News Patrick Reevell