U.S. President Joe Biden, under fire from fellow Democrats to drop his re-election campaign, tested positive for COVID-19 while on a trip to Las Vegas on Wednesday and is 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 the president of UnidosUS, a Latino civil rights organization, said Biden would not be able to speak at a scheduled event due to the diagnosis.

"He is vaccinated and boosted and experiencing mild symptoms, Jean-Pierre said. Minutes after the announcement, the president's motorcade was on the move from taping a radio interview in Las Vegas to the Las Vegas airport. The president was returning to Delaware to self-isolate, the White House said.

Biden had just greeted a couple of dozen people at a Mexican restaurant prior to going behind closed doors for the interview. When Biden was late for the start of the UnidosUS event, the organizer announced he had tested positive.

Biden spent two nights in Vegas on the campaign trail.

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

/"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.