On his final day in Europe, President Joe Biden on Saturday sought to reassure Poland that the United States would defend against any attacks by Russia and he acknowledged that the NATO ally bore the burden of the refugee crisis from the war in neighbouring Ukraine.

“Your freedom is ours," Biden told Poland's president, Andrzej Duda, echoing one of Poland's unofficial mottos.

At the Presidential Palace in Warsaw, the two leaders spoke of their mutual respect and shared goals to end the Russian invasion of Ukraine.

“Although times are very difficult, today Polish-American relations are flourishing," Duda said.

More than 3.7 million people have fled Ukraine since the war began, and 2 million of them are in Poland. Earlier this week the US announced it would take in as many as 100,000 refugees, and Biden told Duda that he understood Poland was “taking on a big responsibility, but it should be all of NATO's responsibility”.

Biden called the “collective defense” agreement of NATO a “sacred commitment," and said the unity of the Western military alliance was of the utmost importance.

“I'm confident that Vladimir Putin was counting on dividing NATO," Biden said about the Russian president. "But he hasn't been able to do it. We've all stayed together.”

With the war entering its second month, European security is facing its most serious test since World War II. Western leaders have spent the past week consulting over contingency plans in case the conflict spreads. The invasion has shaken NATO out of any complacency it might have felt and cast a dark shadow over Europe.

Biden's national security adviser, Jake Sullivan, said a speech that Biden was scheduled to give later Saturday in Poland's capital would outline the “urgency of the challenge that lies ahead” and “what the conflict in Ukraine means for the world, and why it is so important that the free world stay in unity and resolve in the face of Russian aggression.”

Biden's remarks will end a four-day trip that included a series of summits in Brussels. In addition to the meeting with Duda, he stopped by a meeting of American and Ukrainian diplomatic and defense officials for an update on Ukraine's military, diplomatic and humanitarian situation.

Also on the schedule: a stop at stadium where Ukrainian refugees go to obtain a Polish identification number that gives them access to social services such as health care and schools.

The stadium was built in 2012, when Poland and Ukraine hosted the European soccer championship, and was meant as a symbol of how far the two countries had come since the Cold War. More recently, it served as a field hospital for COVID-19 patients.

Biden previewed his closing speech during appearances Friday in Rzeszow.

“You're in the midst of a fight between democracies and oligarchs,” he told members of the US Army's 82nd Airborne Division during a visit to their temporary headquarters. "Is democracy going to prevail and the values we share, or are autocracies going to prevail?"

During a briefing on the refugee response, Biden said “the single most important thing that we can do from the outset" to force Russian President Vladimir Putin to stop the war "is keep the democracies united in our opposition”.

Biden praised the humanitarian effort as being of “such an enormous consequence” given the scope of the crisis, which adds up to the largest flow of refugees since World War II. He appeared to lament that security concerns “understandably” will keep him from visiting Ukraine on this trip.

Duda, who appeared with Biden on Friday, said the refugees are “guests.”

“We do not want to call them refugees. They are our guests, our brothers, our neighbours from Ukraine, who today are in a very difficult situation," he said.

The US has been sending money and supplies to aid the refugee effort. This week, Biden announced $1 billion in additional aid in addition to accepting refugees.

The US and many of its allies have imposed multiple rounds of economic and other sanctions on Russian individuals, banks and other entities in hopes that the cumulative effect over time will force Putin to withdraw his troops.

Biden was scheduled to return to Washington after his Warsaw speech.