U.S. President Joe Biden, under pressure from fellow Democrats to drop his re-election campaign, tested positive for COVID-19 while visiting Las Vegas on Wednesday and is self-isolating after experiencing mild symptoms, the White House said.

White House press secretary Karine Jean-Pierre announced the positive test for the 81-year-old Democrat after Biden cancelled a speech due to the diagnosis.

"He is vaccinated and boosted and experiencing mild symptoms, Jean-Pierre said.

As he boarded Air Force One to depart Las Vegas to recuperate in Delaware, Biden told reporters: "Good, I feel good." But he climbed the stairs slowly, holding the railing tightly and pausing a few steps in and again towards the top.

The illness comes at a crucial time for Biden, who has been losing ground in battleground states against Republican Donald Trump, who is headlining a triumphant convention this week after he survived an assassination attempt on Saturday. The White House said Biden planned to spend a long weekend at his Delaware beach house. It was unclear how long the sickness would keep him for the campaign trail.

Minutes after the announcement, the president's motorcade was on the move to the Las Vegas airport after taping a radio interview in the city.

Biden had greeted a couple of dozen people at a Mexican restaurant prior to going into the radio interview. He was running late to deliver a speech to Latino civil rights group UnidosUS when the organizer, Janet Murguia, announced he had tested positive for COVID.

There were groans in the conference room at the news.

"He said to tell my folks that we're not going to get rid of him that quickly, we're going to have a chance to hear from him in the future directly," Murguia said.

Biden, who had spent two nights in Vegas on the campaign trail, is locked in a battle with some fellow Democrats who worry he is too old to seek re-election and want him to step aside in favor of another candidate.

He has been defiant in the face of the calls to quit the race, telling one interviewer that only the "Lord Almighty" could persuade him to go.

"He will be returning to Delaware where he will self-isolate and will continue to carry out all of his duties fully during that time," said Jean-Pierre.

Biden suffered a blow earlier on Wednesday when a prominent Democratic member of the U.S. House of Representatives, Adam Schiff of California, said it was time for him to "pass the torch" to someone else.

Some 40% of Democratic registered voters said Biden should drop his reelection bid, in a Reuters/Ipsos poll concluded on Tuesday. Some 65% of independent registered voters agreed with them.

Some 58% of Democratic registered voters told the poll they believed Biden is too old to work in government - 70% of independent registered voters agreed.

The White House cited Biden's doctor as saying he had been suffering from upper respiratory symptoms earlier in the afternoon.

"He felt OK for his first event of the day, but given that he was not feeling better, point of care testing for COVID-19 was conducted and the results were positive for the COVID-19 virus," the statement said.

The White House said Biden will be self-isolating in accordance with Centers for Disease Control guidelines.

His symptoms are mild and he has received an initial dose of Paxlovid, the doctor said.

Biden's last bout with COVID began in July 2022. He tested positive on July 21, recovered on July 27, tested positive with a rebound case on July 30 and was finally cleared on Aug 7.