Warsaw, Poland - President Joe Biden praised Ukraine's fierce resistance and NATO's unity as he delivered a major speech in Warsaw ahead of the one-year anniversary of Russia's all-out invasion.

"One year ago, the world was bracing for the fall of Kyiv. Well, I just [came] from a visit to Kyiv and I can report Kyiv stands strong," Biden said on Tuesday in the Polish capital.

To show Washington's commitment to Ukraine's struggle as the country enters its second year of war, Biden had made a dramatic visit to Kyiv on Monday and met with Ukrainian President Volodymyr Zelensky.

"Kyiv stands proud, stands tall and most important stands free," Biden said at Warsaw's Royal Castle, a symbol of the city once largely destroyed in World War II and later rebuilt.

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Biden said Russian President Vladimir Putin's "lust for land and power" had met not only Ukraine's armed forces but the "iron will of Americans."

"Ukraine will never be a victory for Russia. Never," he said.

Biden accused Russian forces of committing "extraordinary brutality" in Ukraine, including "crimes against humanity." He said Russian troops had used rape as a weapon of war, stolen Ukrainian children and bombed train stations, maternity wards and orphanages.

New sanctions will be imposed on Russia "this week," the president said.

The United States and Europe – who provide weapons and financial assistance to Kyiv – do "not seek to destroy or control Russia," Biden said, refuting claims made hours earlier by Russian President Vladimir Putin during a state-of-the-nation speech.

"President Putin chose this war," said Biden. "Putin can end the war with one word."

On Wednesday, Biden plans to meet representatives of other Eastern European NATO countries.

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He reached Poland on Monday evening, after travelling to the Ukrainian capital Kyiv under tight security precautions to meet Zelensky.

There, Biden promised Ukraine further support in its struggle to repel the invading Russian forces.

For Biden, it was the first visit to Ukraine since the war began, and, according to the White House, the first visit by a US president to a war zone without US military being on the ground. Air raid sirens sounded while he was there.

The White House had kept the trip secret until the very end and informed the Russian side of the visit just hours beforehand.

Speaking before Biden, Poland's President Andrzej Duda appealed to NATO member states in Europe to continue supporting Ukraine with weapons.

"I call on all leaders of European NATO countries to show solidarity with Ukraine, to support Ukraine and to provide it with constant military support so that Ukraine's defenders have something to fight with," Duda said.

"Let there be no doubt: The commitment of the United States to our NATO alliance and Article 5 is rock solid," Biden said during his speech in Warsaw.

"Every member of NATO knows it and Russia knows it as well. An attack against one is an attack against all. It's a sacred oath to defend every inch of NATO territory," Biden stated.