As he paid an unannounced and symbolic visit to the capital, Kyiv, President Joe Biden vowed that the US would support Ukraine in its conflict with Russia for "as long as it takes.".
We are absolutely certain that you will continue to win, he said.
Days before the first anniversary of Russia's invasion, Mr. Biden makes his first trip to Ukraine as president. .
President Putin, he claimed, had been "dead wrong" to believe that Russia could outlive Ukraine and its allies in the West.
In their meeting with Ukrainian President Volodymyr Zelensky, Mr. Biden and Mr. Zelensky paid respects to the fallen soldiers in the nine years since Russia annexed Crimea and its proxies seized some of the eastern Donbas region.
According to a statement from the White House, his presence is meant to reaffirm America's "unwavering commitment to Ukraine's democracy, sovereignty, and territorial integrity".
The New York Times claims that President Biden traveled from Poland to Kyiv by train for ten hours.
More military aid for Ukraine, including ammunition and air surveillance radars, is anticipated from Mr. Biden.
Later this week, additional sanctions will be introduced against people and organizations "that are trying to evade or backfill Russia's war machine.".
"The results of this visit will definitely be seen and will definitely be reflected on the battlefield in the liberation of our territories," said Mr. Zelensky. ".
He added that the two leaders had talked about sending additional weapons that had not yet been provided.
Despite Mr. Zelensky's repeated requests, the US and other allies have so far refrained from approving the purchase of F-16 fighter jets.
The US informed Russia about the trip a few hours before President Biden's departure for "deconfliction purposes," according to national security adviser Jake Sullivan, who was in the president's entourage.
On how Moscow had reacted to the news, he declined to comment.
According to White House officials, preparation for the trip has been ongoing for "months," and the decision to go ahead with it was made on Friday.
President Biden and Mr. Zelensky were in the St. Michael's Cathedral in central Kyiv when air raid sirens sounded, adding drama to the most high-profile visit to Ukraine since the war started.
The US president's visit to the country's capital during a conflict in which American soldiers are not engaged is a show of unity at a time when Russia claims that Western support for Ukraine is dwindling, even though other world leaders have made similar trips over the past year.
Ukrainians in Kyiv welcomed the visit.
Roksoliana Gera told the BBC, "I'm so grateful for his support; it means so much to us.
"I admire his bravery for taking on this challenge and for coming to demonstrate the support of the American people. ".
The visit, according to Oleksandra Soloviova, was "a significant sign for the entire world.".
She stated, "For Russia first and foremost, it demonstrates the US supports us and will continue to support us, with sanctions and military hardware.".
According to the US state department, the US is one of Ukraine's biggest allies and has so far announced $24.9bn (£20.6bn) in military assistance.
Mr. Biden stated in January that the US would send 31 battle tanks along with longer-range missiles.
However, there is a widening political gap in the US regarding how much assistance Kyiv should get going forward.
Prior to his three-day trip to Poland, President Biden is in Kyiv.
He will have meetings with the Nato military alliance's Eastern European members there as well as the nation's president, Andrzej Duda.
