Watch: How Zelenskyy Reacted When Biden Called Him "President Putin"

In yet another gaffe amid concerns over his re-election bid, US President Joe Biden today mistakenly introduced Ukraine President Volodymyr Zelenskyy as his Russian rival Vladimir Putin.

"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," he said, referring to Zelenskyy while speaking on the sidelines of the NATO Summit in Washington.

Zelenskyy looked nonplussed, then was seen shaking his head and smiling to himself.

Biden, 81, then quickly corrected himself and said, "President Putin, you're going to beat President Putin, President Zelenskyy. I am so focused on beating Putin."

Zelenskyy, who had by then recovered from his bafflement, responded by saying, "I am better (than Putin)."

"You are a hell of a lot better," Biden replied before Zelenskyy began his address.

Shortly afterward, Biden made another blunder when he referred to his deputy, Kamala Harris, as "Vice President" Trump at a high-stakes news conference.

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"Look, I wouldn't have picked Vice President Trump to be vice president if she was not qualified to be president. So start there," he said as he responded to a question from a journalist about his confidence in Indian-origin leader Kamala Harris.

The latest gaffes come amid growing calls for Biden, who is already the oldest person to ever serve as the US president, to step aside.

The calls have grown louder ever since his poor performance against Donald Trump, 78, in a US presidential debate two weeks ago.

Even members of his Democratic Party are concerned about his ability to win the November 5 election.