They wanted Ukrainian president Volodymyr Zelensky to meet him across the border from Poland, where Mr Biden is currently on an official visit.

Mr Biden, however, was determined that he would go to the Ukrainian capital. The trip had to be organised in hurried secrecy, including a train journey across the Polish border.

Areas in central Kyiv were shut off in the morning, and a news blackout was imposed among government departments.

There were air-raid sirens as Mr Biden arrived with his cavalcade in the capital, adding a sense of drama and giving Mr Biden a glimpse of what citizens across this country have been dealing with for almost 12 months, as we reach the anniversary of Vladimir Putin’s invasion.

He began his visit in solemnity, accompanying President Zelensky on a walk to the memorial, near St Michael’s Cathedral, that honours Ukrainian soldiers who have fallen in the war.

The US president’s visit came on the anniversary of the day during the Maidan protests in 2014 – the demonstrations that led to the overthrow of pro-Moscow president Viktor Yanukovych – when 107 people were killed. They have come to be known as the Heavenly Hundred. 

President Putin’s response to the loss of his ally was to annex Crimea and send troops into the east of Ukraine, instigating the conflict that led to the creation of the separatist republics of Donetsk and Luhansk, and sowing the seeds of the current war.

“It’s good to be back in Kyiv,” Mr Biden declared, adding that this was his eighth visit to Ukraine.

Many of his trips to the country were as Barack Obama’s vice-president, when he built up a close working relationship with the previous Ukrainian president, Petro Poroshenko.

Donald Trump later sought to smear Mr Biden’s reputation by way of his son Hunter Biden’s business interests in the country.

This was, however, Mr Biden’s first visit as president, and the second thing he said – that the US is “here to stay, we are not leaving” – was the note of reassurance he wanted to convey.

He stressed that he was in Ukraine to underline America’s “unwavering and unflagging commitment to Ukraine’s democracy, sovereignty, and territorial integrity”.

Mr Biden added: “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.”

The surprise visit is the first time in modern history that a US leader has visited a war zone where there is not an active American military presence. The White House said it had notified the Kremlin of the visit ahead of time.

At a press conference during the visit, President Biden recalled his conversation with President Zelensky on the night before the invasion.

“It was one year ago this week that we spoke on the telephone, Mr President,” he said.

“Russian planes were in the air, and tanks were rolling across your border. The world was about to change, I remember it vividly.

"I asked you, ‘What is there we can do for you; how can I be of help?’. You said, ‘Gather the leaders of the world, ask them to support Ukraine’.

“I thought it was critical that there not be any doubt, none whatsoever, about US support for Ukraine in the war. The Ukrainian people have stepped up in a way that few people ever have in the past.”

The US president insisted that there continues to be bipartisan support for Ukraine in Washington.

“For all the disagreement we have in our Congress on some issues, there is significant agreement on support for Ukraine,” he said. “It’s not just about freedom in Ukraine. It’s about freedom of democracy at large.”

Mr Biden also paid tribute to Mr Zelensky’s “courage and leadership” in a message he wrote in the guest book at Mariinsky Palace.

President Zelensky called Mr Biden’s visit “an extremely important sign of support for all Ukrainians”.

He welcomed new sanctions due to be imposed on Russia this week, and said the visit was a “clear signal that Russia has no chance” of winning the war and that the US and Ukraine are determined to “together defend our cities and citizens”.

There is apprehension that the Kremlin is trying to replace pro-Western governments in Europe, such as in Moldova, where there is apprehension that an attempt at a coup is being planned.

Demonstrations have taken place in the Moldovan capital, Chisinau, and the country’s leader, Maia Sandu, has said that Moscow intends to “use people from outside the country for violent actions”.

Putin is due to make his annual speech to Russian citizens today in Moscow. This will be followed by a major rally tomorrow, which he may attend.

Russian troops are making slow gains in the east of Ukraine, but are said to be suffering huge losses in troops and equipment.

Meanwhile, China’s Foreign Minister, Wang Yi, arrived in Moscow yesterday amid claims by the US Secretary of State Antony Blinken that Beijing is considering selling arms and ammunition to Russia.

Mr Blinken warned that such a step would have “serious consequences” for Beijing. Russia’s Kommersant newspaper, however, claimed that the trip was to propose a possible peace plan to end the war.

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

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

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

He added: “But I do see an opportunity for China to make a pragmatic assessment of what is happening here, because if China allies itself with Russia, there will be a world war, and I do think that China is aware of that.”