US President Joe Biden has made an unexpected and audacious visit to Ukraine, timed to upstage Russia as the first anniversary of the invasion approaches.

Shrouded in secrecy, the bold trip was the first by an American president in almost 15 years and involved Mr Biden spending 10 hours on a train to Kyiv.

Ukraine President Volodymyr Zelensky said it was “the most important visit in the history of Ukrainian-American relations” as the two leaders walked the streets of the capital.

It comes as Mr Zelensky warned of a third world war if China chose to support Russia, with China’s top diplomat expected to visit Moscow (more below).

The White House said Mr Biden’s surprise trip to a war zone, as air raid sirens wailed in the background, was “unprecedented in modern times” for a US president.

Previous presidential trips to war-torn Iraq and Afghanistan had back-up from the US military.

“It was risky and should leave no doubt in anyone’s mind that Joe Biden is a leader who takes commitment seriously,” said White House communications director Kate Bedingfield.

The visit was timed as Russian President Vladimir Putin is due to make a major speech about the future of the war as the first anniversary approaches on February 26.

The visit sparked anger among Russia’s military pundits who called it a “humiliation” and remarked that Mr Biden could probably visit the frontline unscathed.

“Biden in [Kyiv]. Demonstrative humiliation of Russia,” wrote Russian journalist Sergey Mardan on his Telegram channel.

“I guess there are lunch breaks in a holy war.”

Mr Biden and Mr Zelensky outside St Michael’s Golden-Domed Cathedral in Kyiv. Photo: Getty

Wearing aviation sunglasses and striding alongside Mr Zelensky in battle fatigues, Mr Biden promised to stand with Ukraine as long as it takes.

“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,” Mr 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.”

The US president promised a further $US500 million ($724 million) worth of weaponry, including artillery ammunition, anti-armour systems and air defence radars, plus tighter sanctions on Russia.

‘World war’ warning

Meanwhile Mr Zelensky has warned China against supporting Russia in its war on Ukraine and said doing so would bring on a world war.

Mr Zelensky’s comments were made in a newspaper interview separate to the US president’s trip and as China’s top diplomat Wang Yi is expected to visit Russia.

“For us, it is important that China does not support the Russian Federation in this war,” Mr Zelensky told German daily Die Welt.

“In fact, I would like it to be on our side. At the moment, however, I don’t think it’s possible.”

“But I do see an opportunity for China to make a pragmatic assessment of what is happening here,” he added.

“Because if China allies itself with Russia, there will be a world war, and I do think that China is aware of that.”

US Secretary of State Antony Blinken on Saturday warned top Chinese diplomat Wang Yi of consequences should China provide material support to Russia’s invasion of Ukraine.

After sparring over the shooting down of balloons over the US, Mr Wang has accused the US of violating international norms with “hysterical” behaviour.

Mr Wang suggested European countries “think calmly” about how to end the war and said Beijing would put forward “China’s position on the political settlement of the Ukraine crisis”.

Kremlin spokesman Dmitry Peskov confirmed the planned visit by Mr Wang to Moscow but gave no date for the trip.

“We don’t rule out a meeting between Mr Wang and the [Mr Putin],” Mr Peskov said.

“The agenda is clear and very extensive, so there is lots to talk about.”

A diplomatic source who spoke on condition of anonymity told Reuters earlier that Mr Wang was expected in Moscow shortly and would discuss Chinese ideas for a political settlement of the Ukraine conflict as well as bilateral issues.

Mr Wang, speaking in Budapest on Monday (local time), said China was ready to work with Hungary and other countries to bring hostilities to an end.

Biden upstages Putin

Tweet from @POTUS

Foreign Minister Dmitro Kuleba called Mr Biden’s visit a “victory of the Ukrainian people and president Zelensky” and a clear signal to “the swamp” – Russia – that “no one is afraid of you!”

It was clearly timed to upstage Mr Putin, ahead of his major address on Tuesday (local time) 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.”

Mr Biden and Mr Zelensky together in Kyiv. Photo: Ukrainian Presidency handout

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 Mr Putin victories to tout a year after he launched Europe’s biggest war since World War II.