WASHINGTON: In a major development, US President Joe Biden defiantly insisted Thursday that he will run for another term and win, as a series of verbal gaffes while leading a major summit threw a harsh new spotlight on his fitness.

The 81-year-old president sought to show himself in command as he led the NATO summit in Washington two weeks after a disastrous debate performance raised fears among Democrats that he may lose to Donald Trump.

But Biden mistakenly introduced Ukraine's President Volodymyr Zelensky as his Russian foe Vladimir Putin before quickly correcting himself.

At a high-stakes evening news conference afterward, Biden mistakenly referred to "Vice President" Trump.

But he vowed to stay in the race.

"I think I'm the most qualified person to run for president. I beat him once, and I will beat him again," Biden said of Trump.

Biden, already the oldest person to be elected the first time to the White House, said he was "not in this for my legacy" but to "complete the job I started."

Biden has faced a steady drumbeat of Democrats calling for him to abandon his 2024 candidacy, fearing that Trump is in a position to beat him.

Biden made clear he supported Harris - who as vice president would take over from him, but is also seen by a growing number of Democrats as a stronger candidate at the top of the ticket.

Biden said he would not have picked Kamala Harris, whom he accidentally referred to as Trump, if "she was not qualified to be president."

He also denied reports that he needed to go to bed by 8 pm, a time at which he was still holding his news conference on Thursday.

"Instead of my every day starting at 7:00 am and going to bed at midnight, it would be smarter for me to pace myself a little more," Biden said.

‘Making progress’ on Gaza deal

The US president said his administration was making progress towards a ceasefire deal in war-torn Gaza.

"These are difficult, complex issues. There are still gaps to close. We’re making progress. The trend is positive, and I’m determined to get this deal done and bring an end to this war, which should end now," he told reporters at the White House.

President Biden said he would still be ready to deal with Russian leader Vladimir Putin and China’s Xi Jinping well into a second term as he faced questions over his advancing years and fitness for office.

"I’m ready to deal with them now and three years from now... there isn’t any world leader that I’m not prepared to deal with," he told reporters at the White House.

During the press conference, Biden acknowledged concerns over his mental capacity - saying he was focused on reassuring Democrats amid growing calls for his withdrawal from November´s presidential election.

"I’m determined on running but I think it’s important that I allay fears," he told reporters at a high-stakes press conference.

Mental fitness

President Biden, seeking to allay concerns about his mental fitness as he seeks reelection, said that neurological exams found he is in "good shape."

"I’ve taken three significant and intense neurological exams by a neurologist," Biden said, the most recent in February.

"And they say I’m in good shape."

'Slips of the tongue happen'

Biden's error on Zelensky's name drew gasps from the room but Zelensky, Ukraine's wartime leader against Russia's 2022 invasion, laughed it off.

Fellow leaders at the summit have fielded questions about Biden, and their answers have largely been supportive.

"Slips of the tongue happen, and if you keep a close enough eye on everyone, you will find enough," German Chancellor Olaf Scholz said after the president's latest gaffe.

French President Emmanuel Macron said Biden had appeared "in charge," while British Prime Minister Keir Starmer said he "was on good form."

But key American supporters have moved away from the diplomatic niceties.

Hollywood actor and well-connected Democratic supporter George Clooney called on Biden to exit the race, and party grandee Nancy Pelosi has stopped short of fully backing him.

Around 14 Democratic members of the House of Representatives have openly urged the man who beat Trump four years ago to drop out, along with one Democratic senator.

A poll released on Thursday showed more than half of Democrats say Biden should end his bid for a second term, and two-thirds of Americans believe he should quit the race.

But the former president and the incumbent remain in a dead heat on 46 percent, according to the Washington Post-ABC News-Ipsos survey.

Biden's campaign was however quietly testing the strength of Harris in a potential match-up against Trump, The New York Times reported.

Some of the president's longtime aides are meanwhile discussing how to convince him that he should step aside, the paper also said, citing anonymous sources.

The White House said the report was "unequivocally" false.

The once-talkative Biden has given fewer news conferences than his predecessors, but he was speaking at unusual length Thursday in what his aides called a "big boy" press event.

Biden has called his debate meltdown a "bad night," blaming it on a cold and jet lag.

But Clooney tried to torpedo the narrative that it was a one-off, saying it was "devastating" to admit but the signs were also clear at a June 15 fundraiser in Los Angeles he hosted.

Biden's campaign fought back Thursday with a new ad campaign on the last day of the NATO summit portraying Trump as a "lap dog" of Putin.

NATO allies have also been seeking reassurance about Biden's leadership abilities and over their fears that a return of the isolationist, Putin-praising Trump could spell trouble for the alliance.