US President Joe Biden tested positive for Covid-19 in a "rebound" case, days after ending isolation and treatment, according to his doctor.

White House physician Kevin O'Connor wrote in a memo on Saturday that the President "has experienced no reemergence of symptoms, and continues to feel quite well" but will reinitiate strict isolation procedures.

"This being the case, there is no reason to reinitiate treatment at this time," he added. "But we will obviously continue close observation."

Biden, 79, first tested positive for the coronavirus on July 21 before receiving negative testing results earlier this week, Xinhua news agency reported.

Fully vaccinated and twice boosted, the President took Paxlovid, an antiviral therapy produced by Pfizer and given to patients with Covid-19.

There is potential for so-called "rebound" Covid positivity observed in a small percentage of patients treated with Paxlovid, according to O'Connor.