A defiant Joe Biden insisted Thursday that he will run for another term as US president and beat Donald Trump, as a string of verbal gaffes at a major summit threw a harsh new spotlight on his fitness.

In a high stakes press conference at the end of the NATO summit in Washington, the 81-year-old acknowledged the need to "allay fears" among Democrats but said he was determined not to step aside.

His bid to portray himself as in command was undermined by introducing Ukraine's President Volodymyr Zelensky as his Russian foe Vladimir Putin earlier in the day, and then referring to Vice President Kamala Harris as "Vice President Trump" at the news conference.

Biden's candidacy has been in crisis since a disastrous debate performance against Trump two weeks ago renewed concerns around his age -- and the rare solo press conference, lasting about an hour, was designed to show he still has what it takes.

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

Already the oldest president in US history, Biden said he was "not in this for my legacy" but to "complete the job I started."

The president 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. 

More calls from House representatives came Thursday night after the news conference.

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

'Pace myself'

Amid reports that Biden's campaign was quietly testing Harris's strength in a theoretical match-up against Trump, the president said he would not have picked her 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.

But after blaming his debate debacle on a mixture of jet lag and a cold he admitted it would be "smarter for me to pace myself a little more."

He stressed too that neurological exams showed he was in "good shape" and said he would take another if his doctors recommended one, but they hadn't. 

Biden also fielded a series of foreign and domestic policy questions with detailed if occasionally meandering answers and relatively few slip-ups, though he did mix up Europe and Asia.

With questions swirling about his ability to hold his own against autocratic leaders like Putin or China's Xi Jinping, Biden said he was "ready to deal with them now and three years from now."

Yet his relatively assured performance failed to stop the bleeding, with three more Democratic lawmakers calling for him to quit the race, bringing the total to 17.

Trump meanwhile mocked Biden over his gaffes during the press conference itself. 

"Crooked Joe begins his 'Big Boy' Press Conference with, 'I wouldn't have picked Vice President Trump to be vice president... Great job, Joe!" Trump said on his Truth Social site.

'Big Boy'

The White House had dubbed it a "Big Boy" press conference as it was Biden's first solo, unscripted appearance in eight months -- amid accusations that his aging has been shielded from the public.

The concerns about Biden's health were exacerbated by Biden's error on Zelensky's name, which drew gasps from the room. 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," German Chancellor Olaf Scholz said.

French President Emmanuel Macron said Biden had appeared "in charge" during the summit 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 donor George Clooney called on Biden to exit the race, and party grandee Nancy Pelosi has stopped short of fully backing him.

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.

Biden and Trump remain in a dead heat at 46-47 percent support among voters, respectively, according to the Washington Post-ABC News-Ipsos survey.