US President Joe Biden on Thursday mistakenly referred to Ukrainian President Volodymyr Zelenskiy as Russian President Vladimir Putin before correcting himself at the NATO summit in Washington.

"And now I want to hand it over to the president of Ukraine, who has as much courage as he has determination, ladies and gentlemen, President Putin," Biden said, referring to Zelenskiy.

While correcting himself about two seconds later, Biden added: "President Putin, you're going to beat President Putin, President Zelenskiy. I am so focused on beating Putin."

The room at the summit gasped when Biden misidentified Zelenskiy as Putin. The comments came at an event in the summit during which Biden launched an initiative with allies aimed at supporting Ukraine's security needs.

Zelenskiy responded to Biden's comments by saying, "I am better (than Putin)."

Biden replied: "You are a hell of a lot better," as some in the room laughed, before Zelenskiy began his own address.

Biden has been under intense scrutiny in recent days and has faced doubts, including from members and donors of his own Democratic Party, about his re-election chances after a weak and faltering performance in a debate late last month against Republican former President Donald Trump.

Biden has thus far vowed to push on with his re-election bid and refused to step aside as his party's presidential candidate, while claiming he is best positioned to beat Trump in the November elections.

German Chancellor Olaf Scholz defended Biden on Thursday after the president's mistake. Scholz said: "Slips of tongue happen, and if you always monitor everyone, you will find enough of them."

British Prime Minister Keir Starmer, in a press conference of his own, repeatedly avoided answering the question as to whether Biden was fit enough to run for the U.S. presidency. He instead praised Biden's role in organizing and leading what he said was a successful meeting of the NATO alliance.

Later on Thursday, Biden held a solo press conference at the NATO summit, his first time facing press alone since November, in which he was asked about his gaffe. He responded by saying the NATO summit was successful under his leadership.

"Have you seen a more successful conference?" Biden said to reporters when asked about potential concerns among foreign officials about his fitness for re-election.

At the summit in Washington, NATO members have extended support to Ukraine to combat the Russian invasion that began in February 2022. The United States has been Ukraine's most important partner in military assistance during the war.

Earlier in the day, ahead of a bilateral meeting, Biden told Zelenskiy: "We will stay with you, period."