President Joe Biden, flanked by Poland’s President Andrzej Duda and Samantha Powers of the U.S. Agency for International Development, spoke at a briefing on humanitarian efforts Friday, again calling Russian President Vladimir Putin a "war criminal."

"The single-most important thing that we can do on the outset, is keep the democracies united in our opposition, and our effort to curtail the devastation that is occurring at the hands of a man, who quite frankly, I think is a war criminal. And I think we'll meet the legal definition of that, as well," Biden said.

Biden said he's in Poland to see the "humanitarian crisis" "firsthand," but said he's disappointed he "can't see it firsthand like I have in other places."

"They will not let me … cross the border and take a look at what’s going on in Ukraine," Biden said. "But, you know, I’m eager to hear from you, the humanitarian community, about what you see, what you’re doing, and where you think we go from here."

Since the invasion began on Feb. 24, over 3.6 million people have fled Ukraine, with more than 2.2 million of those refugees going to Poland, according to the United Nations High Commissioner for Refugees.

"Whether it’s food, or a blanket, or cash, or the care for medical teams that we send in, or child welfare specialists, they need it now. They need it as rapidly as we can get it there," Biden said.

-ABC News' Armando Tonatiuh Torres-García

President Joe Biden spoke to members of the 82nd Airborne Division in Jasionka, Poland, Friday, telling them, "What you're doing is consequential -- really consequential."

"What’s at stake" is beyond Ukraine, Biden said.

"What are your kids and grandkids gonna look like in terms of their freedom?" Biden said. "The last 10 years there have been fewer democracies that have been formed than we've lost in the world."

"What you’re engaged in is much more than just whether or not you can alleviate the pain and suffering of the people of Ukraine," Biden said.

Biden commended the troops, saying "the rest of the world looks to us, because, you know, we not only lead by the example of our power, but by the power of our example. And your generation combines both. The rest of the world looks at you and sees who you are. They see you are a multiethnic group of Americans that are in fact together and united in one resolve, to defend your country, and to help those who need help."

In Jasionka, Poland, on Friday, President Joe Biden and Secretary of Defense Lloyd Austin visited American troops, thanking them for working alongside Polish allies.

Biden and Austin first greeted members of the 82nd Airborne Division in a makeshift barbershop.

Biden and Austin then stopped by a mess hall and ended up staying for a slice of pizza.

Biden also shared a story about searching for his son, Beau, in a mess hall in Baghdad, only to find him using his mother's maiden name -- Hunter -- on his fatigues.

"I said, 'Beau, what the hell’s going on?' His name was Beau Biden, and he was a colonel, I mean, a major, excuse me," Biden said.

"And I said, 'What happened?' And he said, 'Dad, with the name Biden, everybody thinks something’s going on. So I’m Hunter.' That was his mother’s maiden name," Biden said.

-ABC News' Molly Nagle

Ukrainian troops have been able to retake towns and defensive positions up to 35 kilometers (about 22 miles) east of Kyiv due to counter-attacks and Russian forces falling back on overextended supply lines, the U.K. Ministry of Defense said Friday in an intelligence update on the situation.

Ukrainian troops are likely to continue to attempt to push Russian forces back along the northwestern axis from the Ukrainian capital toward Antonov Airport in Hostomel, a suburb of Kyiv, according to the U.K. Ministry of Defense.

In southern Ukraine, Russian forces are still attempting to circumvent the densely populated city of Mykolaiv as they look to drive west toward Odesa, with their progress being slowed by logistic issues and Ukrainian resistance, the U.K. Ministry of Defense said.

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