During a visit to Poland, he also pledged America's help amid a humanitarian crisis triggered by Russia's invasion of Ukraine.
“Your freedom is ours," Biden said during a meeting with his Polish counterpart Andrzej Duda at the presidential palace in Warsaw.
He reiterated his stance that NATO's Article 5 guarantee of mutual defence between allies "is a sacred commitment."
Article 5 of the Washington Treaty guarantees collective defence by stating that an armed attack against one NATO member state “shall be considered an attack against them all.”
'We take Article 5 as a sacred commitment'
"We take Article 5 as a sacred commitment, not a throwaway, a sacred commitment that relates to every member of NATO," Biden said.
He added that allies must remain "absolutely, completely thoroughly united."
He told Polish leaders he was confident that Russian President Vladimir Putin "was counting on dividing NATO," but "he hasn’t been able to do it. We’ve all stayed together.”
World should help Poland amid Ukraine refugee crisis: Biden
Biden also said during his meeting with Duda that Poland has borne the brunt of the humanitarian fallout from Russia's war in Ukraine, adding that the world should help the country deal with the refugee crisis.
More than 3.7 million people have fled Ukraine since Russia invaded it more than a month ago, and Poland has welcomed more than 2 million refugees, according to officials.
Biden said in Warsaw on Saturday that Poland was “taking on a big responsibility." He added that "it should be all of NATO's responsibility."
"We have, in our southern border, thousands of people a day--literally, not figuratively--trying to get into the United States, but we believe that we, the United States, should do our part relative to Ukraine as well, by opening our borders to another 100,000 people," Biden told Duda.
Talks with Duda, meeting with refugees
The Polish and US presidents held talks in Warsaw in the early afternoon on Saturday to discuss Russia's war on Ukraine and the West's response to what many say is the worst security crisis in Europe since World War II.
Joe Biden and Andrzej Duda meet in Warsaw on Saturday. Photo: PAP/Marcin Obara
Later in the day, Biden visited a refugee reception centre at Warsaw’s National Stadium together with Polish Prime Minister Mateusz Morawiecki and Warsaw Mayor Rafał Trzaskowski.
He later tweeted: "I visited Ukrainian refugees who have fled to Poland this afternoon. You don’t need to speak the same language to feel the roller-coaster of emotions in their eyes."
He thanked the people of Warsaw for opening their hearts to help.
Biden calls Putin 'a butcher'
During his meeting with refugees who have fled the war in Ukraine to the Polish capital, Biden branded Russian President Vladimir Putin a "butcher," news outlets reported.
"He’s a butcher,” Biden told a reporter when asked what he thought of Putin and the Russian leader's decision to invade Ukraine.
Meeting with Ukrainian government officials
Earlier on Saturday, Biden spoke with Ukraine's Foreign Minister Dmytro Kuleba and Defence Minister Oleksii Reznikov, who were in Warsaw for talks with US Secretary of State Antony Blinken and Defense Secretary Lloyd Austin, according to officials.
Biden to make major speech in Warsaw
In the late afternoon, Biden was expected to make a "major address" to the Polish people outside Warsaw’s historic Royal Castle.
The White House said he would speak about “the united efforts of the free world to support the people of Ukraine, hold Russia accountable for its for its brutal war, and defend a future that is rooted in democratic principles."
Busy two-day trip to Poland
Biden arrived in Poland on Friday afternoon for a busy two-day trip to discuss the international community's response to Russia’s invasion of Ukraine and the humanitarian crisis the war has created, according to officials.
US President Joe Biden touched down in Poland on Friday afternoon for a busy two-day trip to discuss the West's response to Russia’s invasion of Ukraine and the humanitarian crisis the war has created. Photo: PAP/Darek Delmanowicz
His arrival marked the 14th time a US president has visited Poland, local news outlets reported.
Joe Biden's trip marks the 14th time a US president has visited Poland, according to officials. Photo: PAP/Darek Delmanowicz
After his Air Force One plane landed at Rzeszów-Jasionka Airport in the southeast of the country, Biden was greeted by a delegation of officials led by Poland's Defence Minister Mariusz Błaszczak.
The US leader then left the airport to visit American troops from the 82nd Airborne Division stationed in Rzeszów, Polish state news agency PAP reported.
Later that day, Biden met with local charities helping refugees from war-torn Ukraine together with the Polish president.
Polish President Andrzej Duda (right) and US President Joe Biden (left) meet with charity workers helping refugees from Ukraine, in the southeastern Polish city of Rzeszów on Friday, March 25, 2022. Photo: PAP/Łukasz Gągulski
The meeting focused on the humanitarian response to the war in Ukraine, officials told reporters.
Biden announced in a tweet on Friday that the United States "is prepared to provide an additional USD 1 billion for humanitarian assistance and welcome up to 100,000 Ukrainians and others fleeing Russian aggression."
'Strategic nature of Polish-American relations'
In a televised address to the nation ahead of Biden's visit, Poland's Duda said: “The presence of the American leader in our country in those trying days is a very important sign corroborating the strategic nature of Polish-American relations.”
He added: “In the face of a genuine threat, we can see in all clarity that strong American leadership is needed in the world. We see how important the presence and engagement of the United States in Europe are.”
Duda also said that a “secure Poland and secure Europe need more of America," both militarily and economically.
"These will be the topics I will raise in my talks with the United States President,” he announced.
'A frontline and very vulnerable ally'
US National Security Adviser Jake Sullivan has said that Biden's visit comes at a time when Poland "has taken the brunt of the humanitarian impact outside of Ukraine in terms of the refugee flows."
Poland is also where the United States "has surged a significant number of forces to be able to help defend and shore up the eastern flank," Sullivan added.
"Poland has to contend not just with the war in Ukraine but with Russia’s military deployments to Belarus, which have fundamentally changed the security equation there," Sullivan told reporters earlier this week.
"And so, for all of those reasons, we feel that it is the right place for him to go to be able to see troops, to be able to see humanitarian experts, and to be able to meet with a frontline and very vulnerable ally," Sullivan said.
Saturday was day 31 of Russia’s invasion of Ukraine.
Poland on Saturday reported it had welcomed 2.27 million refugees fleeing Russia's invasion of Ukraine.
The Polish president this month signed into law a measure to offer wide-ranging support to Ukrainians escaping the Russian invasion of their country, which began on February 24.
The measure grants them legal residence in Poland and ensures access to education, healthcare and social benefits.
Source: IAR, PAP, TVP Info, president.pl, CNN, Reuters