Russian President Vladimir Putin prepares for a speech. File photo

Following his surprise visit to Kyiv, Biden flew to Poland and on Tuesday will give a speech on how the United States has helped rally the world to support Ukraine and stress American support for NATO's eastern flank.

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Biden, in his trademark aviator sunglasses, and President Volodymyr Zelensky, in green battle fatigues, walked side-by-side to a gold-domed cathedral in Kyiv on a bright winter Monday morning pierced by the sound of air raid sirens.

"When Putin launched his invasion nearly one year ago, he thought Ukraine was weak and the West was divided. He thought he could outlast us. But he was dead wrong," Biden said.

US President Joe Biden attends a press conference in Kyiv on Monday. AFP

"The cost that Ukraine has had to pay is extraordinarily high. Sacrifices have been far too great... We know that there will be difficult days and weeks and years ahead."

Outside the cathedral, burned-out Russian tanks stand as a symbol of Moscow's failed assault on the capital at the outset of its invasion, which began on Feb. 24. Its forces swiftly reached Kyiv's ramparts - only to be turned back by unexpectedly fierce resistance.

Since then, Russia's war has killed tens of thousands of Ukrainian civilians and soldiers on both sides, cities have been reduced to rubble, and millions of refugees have fled. Russia says it has annexed nearly a fifth of Ukraine, while the West has pledged tens of billions of dollars in military aid to Kyiv.

Reuters