WASHINGTON, July 31 (Reuters) - US President Joe Biden again tested positive for COVID-19 on Saturday (Jul 30) and will cancel two planned trips but is experiencing no symptoms and feels "quite well".

Biden, who tested positive for the disease nine days ago but then tested negative twice earlier this week, "will isolate at the White House until he tests negative" and is cancelling travel in the coming days to Delaware and Michigan, the White House said Saturday.

Biden had planned the Michigan trip to tout Thursday's passage of legislation to boost the semiconductor chips industry, which the White House announced earlier Saturday.

Biden's positive test is believed to be "rebound" positivity experienced by some COVID-19 patients, according to White House physician Dr. Kevin O'Connor.

Biden tweeted about his positive case, saying it can happen to a "small minority of folks".

"I’ve got no symptoms but I am going to isolate for the safety of everyone around me. I’m still at work, and will be back on the road soon," he tweeted.

O'Connor said Biden, who is 79, tested negative for the last four days, and there is no plan to reinitiate treatment given his lack of symptoms.

Biden previously described his experience with COVID as mild, saying he was able to continue working while in isolation and attributed his relative ease with the disease to vaccines and other treatments.

O'Connor had previously said Biden would be tested regularly to watch for a potential "rebound" COVID-19 case, which can be experienced by some patients who have been treated with Paxlovid, the drug the president received.