U.S. President Joe Biden steps out of an Air Force One as he arrives to visit Poland, amid Russia's invasion of Ukraine, near Rzeszow, Poland, March 25, 2022. Patryk Ogorzaleki/Agencja Wyborcza.pl via REUTERS
U.S. President Joe Biden steps out of an Air Force One as he arrives to visit Poland, amid Russia's invasion of Ukraine, near Rzeszow, Poland, March 25, 2022. Patryk Ogorzaleki/Agencja Wyborcza.pl via REUTERS Reuters / Patryk Ogorzalek/AGENCJA WYBORCZ

U.S. President Joe Biden said on Saturday that Russia's invasion of Ukraine threatened to unravel global security, and that the world's democracies must prepare for a long fight against autocracy.

"The West is now stronger, more united than it has ever been," Biden told hundreds of Polish elected officials, students and U.S. embassy staff, many holding U.S., Polish and Ukrainian flags.

"We need to steel ourselves for the long fight ahead."

Calling the fight against Vladimir Putin a "new battle for freedom," Biden said Putin's desire for "absolute power" was a strategic failure for Russia and a direct challenge to a European peace that has largely prevailed since World War Two.

The remarks at Warsaw's Royal Castle came as Biden made fresh security promises to Ukraine and branded Putin a "butcher" during a meeting with refugees who have fled the war in Ukraine to the Polish capital.

Biden, who took office last year after a violently contested election, vowed to restore democracy at home and unite democracies abroad to confront autocrats including the Russian president and China's leader Xi Jinping.

Putin's Feb. 24 invasion of Ukraine has tested that promise and threatened to inaugurate a new Cold War three decades after the Soviet Union unravelled.

The U.S. president is wrapping up three days of emergency meetings in Europe with the G7, European Council and NATO aimed at building a unified approach to thwart Putin.

Earlier in the day, Biden dropped in on a meeting that Ukraine's foreign and defense ministers held with U.S. officials.

"President Biden said what is happening in Ukraine will change the history of the 21st century, and we will work together to ensure that this change is in our favour, in Ukraine's favour, in the favour of the democratic world," Ukraine's foreign minister, Dmytro Kuleba, said on the country's national television service.

After a separate meeting with Polish President Andrzej Duda, Biden reiterated Washington's "sacred" commitment to security guarantees within NATO, of which Poland is a member.

Ukraine is not a member of the Western military alliance, and the United States is wary of getting dragged into direct confrontation with nuclear-armed Russia, but Washington has pledged to defend every inch of NATO territory.

Ukraine received additional security pledges from the United States on developing defence co-operation, Kuleba told reporters, while Defence Minister Oleksii Reznikov expressed "cautious optimism" following the meeting with Biden.

REFUGEES

In Warsaw, Biden also visited a refugee reception centre at the national stadium. People, some waving Ukrainian flags, lined the streets as his motorcade wound its way towards the stadium.

After being greeted by celebrity chef Jose Andres, Biden talked to refugees who had gathered to receive food from the World Central Kitchen NGO, asking their names and hometowns and posing for pictures with some.

More than 2 million people have fled the war to Poland. Altogether, about 3.8 million have left Ukraine since fighting began.

Asked about the impact that Putin's decision to invade Ukraine had had on the Ukrainian people, Biden said the Russian leader was a "butcher."

Russia's TASS news agency quoted a Kremlin spokesman as saying Biden's latest comments about Putin narrowed the prospects for mending ties between the two countries.

Putin calls Russia's military actions in Ukraine a "special military operation" to demilitarise and "denazify" the country. Russia denies targeting civilians.

Standing outside the stadium, Hanna Kharkovetz, a 27-year-old woman from the northern Ukrainian city of Kharkiv, expressed frustration that the world was not doing more to help.

"I don't know what he wants to ask us here. If Biden went to Kyiv ... that would be better than speaking here with me," she said as she waited to register her mother for a Polish national ID number.

The invasion of Ukraine has tested NATO and the West's ability to unite.

Poland was under communist rule for four decades until 1989 and was a member of the Moscow-led Warsaw Pact security alliance. It is now part of the European Union and NATO.

The rise of right-wing populism in Poland in recent years has put it in conflict with the EU and Washington, but fears of Russia pressing beyond its borders has drawn Poland closer to its Western allies.

Biden's election put the nationalist Law and Justice government in an awkward position as it had set great store in its relationship with his predecessor, Donald Trump.

But as tensions with Russia rose before it invaded Ukraine, Duda appeared to seek to smooth relations with Washington. In December, he vetoed legislation that critics said aimed to silence a U.S.-owned 24-hour news broadcaster.

Biden and Duda were expected in their meeting to address a disagreement over how to arm Ukraine with warplanes, and other security guarantees.