US President Joe Biden has suffered another embarrassing gaffe at the start of a highly-anticipated press conference referring to “Vice President Trump” amid growing calls for him to drop out of the election.

Biden, 81, spoke in Washington DC at a NATO summit on Thursday, about 7.30pm local time, where he quickly mixed-up Vice President Kamala Harris with political rival, Donald Trump.

WATCH THE VIDEO ABOVE: Biden defends candidacy despite another epic gaffe.

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Biden was asked by Reuters reporter Jeff Mason: “What concerns do you have about Vice President Harris’s ability to beat Donald Trump? If she were at the top of the ticket?”

“Look, I wouldn’t have picked Vice President Trump to be Vice President, but I think she was not qualified to be president,” Biden said to reporters at the press conference.

Just hours earlier, Biden made another humiliating gaffe by calling Ukrainian leader Volodymyr Zelensky “Putin” during a press conference at the NATO summit.

Despite growing calls for him to drop out of the race for re-election, Biden said he was determined to face-off with his rival Donald Trump in November.

“I think I’m the most qualified person to run for president. I beat him once, and I will beat him again,” Biden said.

“Secondly, the idea that senators and congressmen (who) are running for office, worry(ing) about the ticket is not unusual. There are at least five presidents, running or incumbent presidents, who had lower numbers than I have now.”

After batting away several questions about his fitness to continue to stand against Donald Trump in presidential election race, Biden pivoted to US domestic issues.

“Think about where we are economically relative to the rest of the world. Name me a world leader who wouldn’t want to trade places with our economy. We created over 800,000 manufacturing jobs, 1.5 million,” Biden said.

Biden then appealed to blue-collar workers in the US, complaining of “corporate greed.”

“Working-class people still have need help. Corporate greed is still at large. Their prices. The corporate profits have doubled since the pandemic. They’re coming down.”

Biden praised the NATO mutual defence treaty, commonly referred to as Article Five. He took aim at Trump’s continued demoralisation of the alliance, which has served as a bulwark against Russia.

The presidential nomination is set to be formalised in mid-August, with a roll-call vote at the Democratic National Conference in Chicago.

Biden has repeatedly said he will stay in the race, despite Hollywood Democrat supporters George Clooney and Michael Douglas, as well as powerful House Democrats questioning his future.

He defended his campaigning amid claims he had been forced to take on a lighter schedule to conserve his energy, following his dismal debate performance against Trump.

Biden is an establishment figure in Washington politics, having spent 36 years as a US senator, eight years as US vice president under Barack Obama, and four years as president.