US President Joe Biden walked around central Kyiv on an unannounced visit on Monday, promising to stand with Ukraine as long as it takes, on a trip timed to upstage the Kremlin ahead of the one-year anniversary of Russia's invasion.

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

"When (Russian President Vladimir) 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.

"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, 2022. 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.

"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.

Biden travelled to Ukraine's capital by overnight train from Poland, arriving after roughly 10 hours at 8am on Monday, before returning there the same way, leaving just after 1pm, according to a White House pool report by a Wall Street Journal reporter.

Biden arrived late on Monday in Warsaw, where he is scheduled to meet Poland's President Andrzej Duda, along with other leaders of countries on Nato's eastern flank, the following day.

While Biden was in Kyiv, the State Department announced a further US$460 million in US aid to Ukraine, including US$450 million worth of artillery ammunition, anti-armour systems and air defence radars, and US$10 million for energy infrastructure.

The European Union's foreign policy chief, Josep Borrell, said the bloc would approve more sanctions before the anniversary of the conflict, which Russia says is a "special military operation".

Russia was notified before Biden's departure, officials in Washington and Moscow said, apparently to avoid the risk of an attack on Kyiv while he was there.

The trip took place a day before Putin was 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 the trans-Atlantic military alliance 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.

Winter offensive

Russia has sent thousands of conscripts into Ukraine for a winter offensive but 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 signal of diplomatic support on Monday, with Chinese Foreign Minister Wang Yi expected for talks. In public, China has 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".

A diplomatic source speaking on condition of anonymity told Reuters that Wang Yi would discuss Chinese ideas for a political settlement of the war. Ukraine says any diplomatic solution requires the withdrawal of Russian forces from its territory.

Russia is trying to secure full control of two eastern provinces forming Ukraine's Donbas industrial region. It has launched assaults at locations running from Kreminna in the north down to Vuhledar in the south, securing its biggest gains around the mining city of Bakhmut.

Kyiv, which is absorbing a major influx of Western weaponry in the coming months for a planned counteroffensive, has lately stuck mainly to defence on the battlefield, claiming to be inflicting huge casualties on the assaulting Russian forces.

Three civilians were killed in Russian shelling on Monday, regional Ukrainian officials said - one in the Donetsk region and two in the Kherson region.

Wagner frustrated

Britain's Ministry of Defence said Russia's casualties included two elite brigades of thousands of marines probably rendered "combat ineffective" by losses sustained in failed attempts to storm Vuhledar.

"The Russian forces are likely under increasing political pressure as the anniversary of the invasion draws near," it said, predicting Moscow may claim to have captured Bakhmut regardless of the situation on the ground. "If Russia's spring offensive fails to achieve anything, then tensions within the Russian leadership will likely increase."

In a sign of such dissent, Yevgeny Prigozhin, a Putin ally whose Wagner private army has sent thousands of criminals recruited from prison into battle around Bakhmut, accused unidentified Russian officials of sabotaging his force by withholding weapons.

Inside Vuhledar, constant explosions shook the ruins. A pensioner emerged from the cellar where she lives with her dog and showed a Reuters journalist around the rubble of her flat above, where a shell had blasted through the wall.

She said she had been saved when the room was hit because a fridge had fallen on top of her. A neighbour's daughter found her and dragged her out.

"Scary is not the word. It is terrifying," she said.

- Reuters