US President Joe Biden on Tuesday said NATO was stronger than ever during a visit to Poland ahead of the first anniversary of Russia's invasion of Ukraine.

"A year later, I would argue that NATO is stronger than it's ever been," Biden told his Polish counterpart Andrzej Duda, a day after making a surprise trip to Kyiv.

"I can proudly say that our support for Ukraine is unwavering," he said during the bilateral talks shown in part on Polish state television.

Biden called Poland's support for neighbouring Ukraine and its citizens "truly extraordinary".

Duda said that thanks to Biden, "we can see that America can keep the world order".

Biden is due to deliver a speech in Warsaw later Tuesday.

On Wednesday, he will meet with the leaders of nine countries on NATO's eastern flank.