U.S. President Joe Biden arrived at Poland's Rzeszow-Jasionka Airport Friday afternoon, where he will get a firsthand look at the international efforts to help some of the millions of people fleeing Ukraine.

Biden was greeted on the tarmac by four U.S. commanding generals. While in Rzeszow on Friday, Biden will receive a briefing on the humanitarian situation and meet with humanitarian aid groups as well as service members of the U.S. Army's 82nd Airborne Division.

He will later travel to Warsaw, where he will meet with Polish President Andrzej Duda as well as refugees from Ukraine on Saturday.

U.S. President Joe Biden's support of NATO battlegroups on the eastern flank stems from the belief that Russia's attacks in Ukraine have the capacity to put the alliance's territories at risk, according to White House national security adviser Jake Sullivan.

"We do believe Russian aggression in Ukraine shows a willingness by the Russians to disregard international borders and to disregard the basic rules of the road of the international community that have been built in sustained over the course of seven decades," Sullivan told reporters aboard Air Force One on Friday.

"It is important in this moment to send a clear message to Russia that the United States and NATO will defend every inch of NATO territory and to deter any thinking that Putin might have about further Russian aggression into NATO."

Sullivan noted that Belarus' willingness to station Russian troops on its soil, in particular, has a "significant impact particularly on our NATO allies in the Baltics and Poland."

Sullivan said Biden has made clear that any diplomatic agreement reached is one that Ukraine will have to determine for itself, meaning Washington is not going to push or pressure Kyiv into any outcome.

Currently, Washington's priority is to ensure Ukraine has the capabilities to defend itself as Russian forces continue pushing forward, Sullivan told reporters.

-ABC News' Armando Garcia

Pope Francis will consecrate Russia and Ukraine to the Immaculate Heart of Mary on Friday, inviting people around the world to join him in the prayer.

"This Act of Consecration is meant to be a gesture of the universal Church, which in this dramatic moment lifts up to God, through His Mother and ours, the cry of pain of all those who suffer and implore an end to the violence, and to entrust the future of our human family to the Queen of Peace," Francis said in a statement.

He also called for an end to the violence.

Consecration is an act of surrender in which the pope recognizes both Russians and Ukrainians as children of god, and entrusts them to Mary’s care, Father Alexandre Mello, the secretary of the Vatican’s Dicastery for the Laity, Family, and Life, told Crux.

Mello also said consecration aims to build bridges as the prayer's goal is to have a healing effect and remind Russians and Ukrainians of their shared roots and identities as children of the same God.

The ceremony is tied to the Marian apparitions in Fatima, Portugal in 1917, in which many Catholics believe the Virgin Mary appeared to three children, asking that the pope consecrate Russia to her immaculate heart.

The Act of Consecration to the Immaculate Heart of Mary will be prayed during the Lenten penitential service in St. Peter's Basilica in Rome which begins at 5 p.m. local time. The pope will start the prayer at around 6:30 p.m. local time and has asked all Catholic Bishops and priests to join him spiritually.

U.S. bishops, including Cardinal Wilton Gregory, the archbishop of Washington, D.C., and Cardinal Blase Cupich, the archbishop of Chicago, announced they will be holding consecration ceremonies on Friday.

Pope Emeritus Benedict XVI said he will join in the prayer from his converted monastery in Vatican City, where he has lived since he resigned.

-ABC News' Phoebe Natanson

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

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