US President Joe Biden stated Saturday in Poland that Russia's President Vladimir Putin "cannot remain in power" words that a White House official later clarified was intended to brace the world's democracies for an extended battle over Ukraine, not to support regime change in Russia.

Biden's remarks on Saturday, which followed a previous speech labeling Putin a "butcher," were a significant escalation of the US response to Moscow's invasion of Ukraine.

Biden invoked Poland's four decades behind the Iron Curtain in a significant talk made in Warsaw's Royal Castle, attempting to make the case that the world's democracies must quickly confront an autocratic Russia as a threat to global security and democracy.

However, comment after the address highlighted the prospect of an escalation by Washington, which has refrained from direct military intervention in Ukraine and has stated explicitly that it opposes regime change.

"For God's sake, this man cannot remain in power," Biden told a Warsaw crowd following his condemnation of Putin's month-long war in Ukraine.

According to a White House official, Biden's words did not signal a change in Washington's approach.

"The President's point was that Putin cannot be allowed to exercise power over his neighbours or the region," the official explained. "He was not discussing Putin's power in Russia, or regime change."

In response to a question regarding Biden's remark, Kremlin spokesperson Dmitry Peskov told Reuters: "That is not a decision for Biden to make. Russians elect their president."

Biden described the fight against Putin as a "new battle for freedom," claiming that Putin's ambition for "absolute power" was a strategic failure for Russia and a direct threat to the peace that has mainly persisted in Europe since World War II.

"The West is now stronger, more united than it has ever been," Biden stated. "This struggle, too, will not be won in a matter of days or months. We must brace ourselves for the long battle ahead."

The address followed three days of discussions in Europe with the G7, the European Council, and NATO allies. It occurred nearly concurrently with rocket attacks on the western Ukrainian city of Lviv, located about 60 kilometers (40 miles) from the Polish border.

"Their brave resistance is part of a larger fight for essential democratic principles that unite all free people," Biden remarked. "We are on your side. Period."

Biden stated in his speech that NATO is a defensive security alliance that has never wanted Russia's demise and that the West has no wish to harm the Russian people, even though its sanctions threaten to devastate their economy.

Poland was communist for four decades until 1989 and was a member of the Warsaw Pact security alliance commanded by Moscow. It has since become a member of the European Union and NATO.

Poland's growth of right-wing populism has pitted it against the EU and Washington in recent years, but fears of Russia expanding beyond its borders have pushed Poland closer to its Western allies.

Biden told a crowd of Americans, Poles, and Ukrainians that the West is working together because of the "gravity of the threat" to global peace.

"The battle for democracy could not conclude and did not conclude with the end of the Cold War," Biden stated. "Over the last 30 years the forces of autocracy have revived all across the globe."

In Warsaw, reactions were varied. Mykyta Hubo, a Dnipro-born Ukrainian who has lived in Poland for some years, described the speech as "ordinary": "Lots of talk, little action," he observed.

Pawel Sterninski, who traveled nearly three hours from other parts of Poland to Warsaw to hear Biden, arrived draped in a US flag.

"The US cannot truly act militarily for fear of sparking a third world war. Putin is unpredictably unpredictable. When nuclear weapons are threatened, it just takes a little period for the threat to escalate into a worldwide conflict."

Earlier in the day, Biden had a meeting with Ukraine's foreign and defense ministries. He made additional, unnamed security commitments for the development of defense cooperation, according to Ukraine's foreign minister, Dmytro Kuleba.

Biden also visited a refugee processing center at Warsaw's national stadium. Over 2 million individuals have fled to Poland due to the war. Since the violence began, around 3.8 million people have fled Ukraine.

Putin has described Russia's military intervention in Ukraine as a "special military operation" aimed at demilitarizing and "denazify" the country.