US President Joe Biden tested positive for Covid with mild symptoms Wednesday, shortly after conceding he would consider dropping his reelection bid if doctors diagnosed him with a serious medical condition.

The 81-year-old Democrat gave reporters the thumbs up and said "I feel good" as he cut short a trip to Las Vegas and flew to his beach home in Delaware to go into isolation, which will take him off the campaign trail for days.

Biden thanked well-wishers on X, adding that "I will be isolating as I recover, and during this time I will continue to work to get the job done for the American people."

The infection comes at a critical moment for Biden's campaign, with the president seeking to show he is up to the job after a disastrous debate performance against rival Donald Trump sparked concerns about his health and calls from some Democrats for him to step aside.

It is also the latest development in a tumultuous few days in an already frenetic White House race that saw Trump survive an assassination attempt at a campaign rally.

Biden was forced to cancel a speech to a union representing Latino workers who will be crucial for his election bid, having attended a campaign event earlier in the day and given a radio interview.

His spokeswoman Karine Jean-Pierre said Biden was vaccinated and boosted, was now taking the Covid medication Paxlovid and "continues to carry out the full duties of the office while in isolation."

White House doctor Kevin O'Connor said Biden had complained of suffering from a runny nose, a cough and "general malaise," and after attending the campaign event he tested positive.

"His symptoms remain mild," O'Connor said, adding that Biden's respiratory rate, temperature and blood oxygen levels all remained normal.

Biden was seen walking from his limousine to his plane at Las Vegas without a mask. "Good," he said when asked how he felt, "I feel good."

Janet Murguia, the president of the Unidos union for Latino workers, told the crowd about the diagnosis shortly before the White House announcement.

- 'Pass the torch' -

People waiting for the speech said Biden's health did not worry them despite the Covid diagnosis.

"I think he's strong and he's going to recover soon," Anne Vilagut told AFP.

But Biden's illness comes as concerns over the fitness of the oldest US president in US history reach fever pitch.

Asked what could make him rethink his presidential bid, Biden told the Black media outlet BET in an interview taped Tuesday in Las Vegas: "If I had some medical condition that emerged, if somebody, if the doctors came and said 'you've got this problem, that problem.'"

Biden has so far refused to drop out, and blamed his debate debacle, when he appeared tired and confused, on a bad cold and jet lag.

But US broadcaster ABC News reported Wednesday that Democratic Senate Majority Leader Chuck Schumer had told Biden over the weekend that it would be "better for the country if he were to bow out," in what would be a fatal blow.

A spokesperson for Schumer played down the report, saying: "Unless ABC's source is Senator Chuck Schumer or President Joe Biden the reporting is idle speculation."

"Leader Schumer conveyed the views of his caucus directly to President Biden."

The Washington Post and New York Times meanwhile reported that both Schumer and House Democratic leader Hakeem Jeffries had warned Biden that his candidacy puts the party's electoral prospects at risk.

White House spokesman Andrew Bates pushed back in a statement, saying: "The President told both leaders he is the nominee of the party, he plans to win, and looks forward to working with both of them to pass his 100 days agenda to help working families."

Earlier on Wednesday, Representative Adam Schiff of California became the highest-profile Democrat to publicly urge Biden to "pass the torch."

"A second Trump presidency will undermine the very foundation of our democracy, and I have serious concerns about whether the President can defeat Donald Trump in November," Schiff said in a statement to the Los Angeles Times.

Biden insists that Democratic voters support him, but a poll by the Associated Press and NORC Center for Public Affairs Research said Wednesday that nearly two-thirds want him to step aside.