Joe Biden has mistakenly referred to Volodymyr Zelenskyy as "President Putin" at the NATO summit - just hours after his allies warned his chances of winning the US election are zero.
The US president was speaking in Washington at the end of the three-day summit and made the error while he was introducing the Ukrainian president.
"Ladies and gentlemen, President Putin," Mr Biden said, referring to Mr Zelenskyy before correcting himself.
"President Putin? I'm going to beat President Putin, President Zelenskyy. I am so focused on beating Putin," he added.
"I am better," joked Mr Zelenskyy in reply.
Latest updates from NATO summit
Mr Biden's misstep at the summit comes just hours after some of his closest allies said they now see his chances of winning the US election as zero and that he will "never recover" from concerns about his mental fitness.
Several allies, including three directly involved in efforts to re-elect him as US president, told Sky News' partner network NBC News that they believe he should drop out of the race.
Biden campaign 'quietly assessing Kamala Harris'
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The Biden campaign is "quietly assessing the viability of vice-president Kamala Harris's candidacy against Donald Trump in a new head-to-head poll", NBC News also reported.
Mr Biden has faced a torrent of questions around his suitability to run over the last two weeks following a disastrous debate against opponent former president Trump.
The White House was also forced to deny he was being treated for Parkinson's disease following reports a specialist doctor had visited the president several times in the past year.
Mr Biden's doctor released a letter on Tuesday saying he has shown no symptoms of Parkinson's or any other neurological disease.
'He needs to drop out'
The allies who spoke to NBC News said the sentiment that Mr Biden should allow another Democrat to fight the presidential election is widespread and includes aides, operatives and officials tasked with guiding his campaign to victory.
One Biden campaign official told NBC News: "He needs to drop out."
A second person working to elect him said: "No one involved in the effort thinks he has a path."
A third close to the campaign said the current situation was unsustainable and they couldn't see how Mr Biden could win.
All spoke on condition of anonymity because they did not want to be seen as further damaging a candidate they appreciate for his victory over Trump in 2020 and his policy wins in the White House.
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Separately, a longtime Democratic presidential campaign strategist told Reuters news agency: "We have this window and the White House is just running out the clock, which is so selfish. We're all waiting around for Joe Biden to f*** up again, which is not a great position to be in."
Two others close to Mr Biden said they had not given up all hope of a turnaround but they saw it as an increasingly unlikely outcome, and they thought defeating Trump in November should take precedence over backing the president.
"The question for me, and a lot of us, is: Who is the best person to beat Donald Trump?" another person working to elect Mr Biden said. "There are a lot of us that are true blue that are questioning our initial thoughts on that."
However, Biden campaign spokesperson TJ Ducklo told NBC News this was "patently false" and the team "stands with the president".
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The White House has insisted Mr Biden will continue to run and "turn the page" on the debate, despite some opinion polls showing Trump's lead widening and swing states leaning Republican.
Officials and allies have previously said Mr Biden's decision to run for re-election at 81 years old centred around his belief he was the best Democratic candidate to beat Trump.
He was said to be skeptical of the ability of his vice-president Ms Harris to win over middle-of-the road voters, especially in critical swing states, according to Reuters news agency sources.
