US President Joe Biden tested positive for Covid during a campaign trip to Las Vegas on Wednesday and is suffering from mild symptoms, the White House said.

"I feel good," 81-year-old Biden said, giving reporters the thumbs up as he leaned out of his limousine dubbed "The Beast," before boarding Air Force One to head for isolation at his beach home in Rehoboth, Delaware.

The announcement came minutes after the head of a Latino union said Biden had called to apologise that he could no longer address its members because he had been diagnosed.

It also comes as Biden is under growing pressure to drop his reelection bid against Donald Trump after a disastrous debate performance sparked concerns about his age and health.

"Earlier today following his first event in Las Vegas, President Biden tested positive for Covid-19," spokeswoman Karine Jean-Pierre said in a statement.

"He is vaccinated and boosted and he is experiencing mild symptoms. 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."

Biden's doctor said he was suffering from a runny nose and a "non-productive cough" along with "general malaise," the White House said in an accompanying note.

He was now receiving the Covid drug Paxlovid, it added.

"His symptoms remain mild, his respiratory rate is normal at 16, his temperature is normal at 97.8 and his pulse oximetry is normal at 97 percent," it added.

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

"I was just on the phone with President Biden, and he shared his deep disappointment at not being able to join us this afternoon," she said.

"The President has been at many events, as we all know, and he just tested positive for Covid. So of course, we understand that he needs to take the precautions that have been recommended." (AFP)