Biden, in aviator sunglasses, strode side-by-side with President Volodymyr Zelenskiy, in green battle fatigues, through central Kyiv to a gold-domed cathedral, on a bright winter morning pierced by 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 during Monday's visit.

"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, burnt-out Russian tanks had been placed as a symbol of Moscow's failed assault on the capital at the outset of its invasion, when its forces swiftly reached the city's ramparts only to be turned back by unexpectedly fierce resistance.

Since then, tens of thousands of Ukrainian civilians and soldiers on both sides have died, cities have been reduced to rubble and millions of refugees have fled. Russia claims to have annexed nearly a fifth of Ukraine, while the West has committed tens of billions of dollars in military aid to Kyiv.

The US president promised a further $US500 million ($A724 million) worth of weaponry, including artillery ammunition, anti-armour systems and air defence radars, plus tighter sanctions on Russia.

"This visit of the US president to Ukraine, the first for 15 years, is the most important visit in the entire history of Ukraine-US relations," Zelenskiy said.

Foreign Minister Dmitro Kuleba called the visit a "victory of the Ukrainian people and president Zelenskiy" and a clear signal to "the swamp" - Russia - that "no one is afraid of you!"

It was clearly timed to upstage Russia's President Vladimir Putin, due to make a major address on Tuesday setting out aims for the second year of what he now calls a proxy war against the armed might of Washington and NATO.

"Of course for the Kremlin this will be seen as further proof that the United States has bet on Russia's strategic defeat in the war and that the war itself has turned irrevocably into a war between Russia and the West," said Tatiana Stanovaya, a Russian political analyst.

"Tomorrow's address was expected to be very hawkish, targeted at a demonstrative rupture of relations with the West. Now additional edits could be introduced to make it even tougher."

The anniversary has taken on more than symbolic significance, becoming what the West views as the principal motivation for the war's deadliest phase, with Moscow hurling thousands of conscripts and mercenaries into a winter offensive.

Russia has secured only scant gains so far in assaults in frozen trenches up and down the eastern front in recent weeks. Kyiv and the West see it as a push to give Putin victories to tout a year after he launched Europe's biggest war since World War Two.

Moscow received its own apparent signal of diplomatic support on Monday, with Chinese Foreign Minister Wang Yi expected in the Russian capital for talks. In public, China has so far remained neutral over the conflict despite signing a "no limits" friendship pact with Russia weeks before the invasion.

Washington has said in recent days it is concerned Beijing could begin supplying Moscow with arms. Chinese Foreign Ministry spokesperson Wang Wenbin said the United States was "in no position to make demands of China", and China's "comprehensive collaborative partnership with Russia" was a matter for two independent states.

Australian Associated Press