President Biden in a briefing in Warsaw, Poland, lamented that "they will not let me" cross the border into Ukraine during his trip to Europe amid Russia’s war on the country.

Biden, appearing with Polish President Andrzej Duda on Friday, said he was there to see the humanitarian crisis "first hand."

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"Part of my disappointment is that I can’t see it first hand like I have in other places," Biden said Friday.

President Biden, with Polish President Andrzej Duda, participate in a roundtable on the humanitarian response to the Russian invasion of Ukraine, Friday, March 25, 2022, in Jasionka, Poland. (AP Photo/Evan Vucci)

President Biden, with Polish President Andrzej Duda, participate in a roundtable on the humanitarian response to the Russian invasion of Ukraine, Friday, March 25, 2022, in Jasionka, Poland.  (AP Photo/Evan Vucci)

"They will not let me, understandably, I guess, cross the border and take a look at what’s going on in Ukraine," Biden said.

It is unclear who Biden was referring to, but the president is likely not traveling into Ukraine due to security reasons.

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Earlier this month, prime ministers Mateusz Moravetsky of Poland, Petr Fiala of the Czech Republic and Janez Jansa of Slovenia traveled to Kyiv to meet with Ukrainian President Volodymyr Zelensky and Prime Minister Denys Shmyhal.

President Biden visits with members of the 82nd Airborne Division at the G2A Arena, Friday, March 25, 2022, in Jasionka, Poland. (AP Photo/Evan Vucci)

President Biden visits with members of the 82nd Airborne Division at the G2A Arena, Friday, March 25, 2022, in Jasionka, Poland.  (AP Photo/Evan Vucci)

Meanwhile, Biden thanked humanitarian aid workers for their work to help in the "suffering that’s taking place" at their "doorstep."

"You’re the ones at risk," Biden said. "In some cases, your lives, and risking all, you know, to try to help, and the American people are proud to support your efforts." 

Poland has been directly impacted by the Ukraine-Russia war, having taken in more than 2 million of Ukraine's 3 million refugees. Poland also houses U.S. troops.

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The White House said this week that the U.S. will welcome up to 100,000 Ukrainians and others seeking to escape from Russia's invasion and will impose new sanctions on Russian individuals and companies. 

President Biden speaks to members of the 82nd Airborne Division at the G2A Arena, Friday, March 25, 2022, in Jasionka, Poland. (AP Photo/Evan Vucci)

President Biden speaks to members of the 82nd Airborne Division at the G2A Arena, Friday, March 25, 2022, in Jasionka, Poland.  (AP Photo/Evan Vucci)

According to senior administration officials, the Biden administration is looking into multiple methods for those fleeing Ukraine to enter the U.S., including the refugee admissions program, as well as parole and immigrant and nonimmigrant visas.

Biden said Friday that the "single most important thing" that the U.S. and NATO allies can do is "keep democracies united in our opposition and our effort to curtail the devastation that is occurring at the hands of a man, I, quite frankly, think is a war criminal."

"I think he will meet the legal definition of that as well," Biden said.

U.S. Secretary of State Antony Blinken this week began officially calling the actions by certain members of Russia's forces in Ukraine "war crimes."

Blinken's comments come nearly a week after he and Biden said they personally believe that Russian President Vladimir Putin is a war criminal.

"Today, I can announce that, based on information currently available, the U.S. government assesses that members of Russia’s forces have committed war crimes in Ukraine," a statement from Blinken read. "Since launching his unprovoked and unjust war of choice, Russian President Vladimir Putin has unleashed unrelenting violence that has caused death and destruction across Ukraine."

US SECRETARY OF STATE BLINKEN SAYS RUSSIA IS COMMITTING WAR CRIMES IN UKRAINE

He said on Wednesday that the U.S. government assessment of war crimes being committed comes after a "careful review" of information from public and intelligence sources, stating that the United States would continue to track reports of war crimes.

Blinken also said that the State Department has seen credible reports of "indiscriminate attacks" by Russian forces against Ukrainians, adding that they have destroyed schools, hospitals, critical infrastructure, apartment buildings and more.

He said that many of these locations struck by Russian forces are "clearly identifiable" as being used by civilians.

He cited the strike on a Mariupol theater on March 16, stating that it was clearly marked with the Russian word for "children."

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"This includes the Mariupol maternity hospital, as the UN Office of the High Commissioner for Human Rights expressly noted in a March 11 report. It also includes a strike that hit a Mariupol theater, clearly marked with the word ‘дети’ — Russian for ‘children’ — in huge letters visible from the sky," Blinken said.

Since the beginning of Russia's invasion of Ukraine, the United Nations Office of the High Commissioner for Human Rights estimates that 977 people have been killed and 1,594 have been injured, but that number is likely significantly higher due to unconfirmed deaths and injuries.

Fox News' Adam Sabes contributed to this report.