US President Joe Biden tested negative for COVID-19 for a second day in a row and will end his isolation, his doctor announced on Sunday.

"This morning, the President's SARS-CoV-2 antigen testing was negative for a second consecutive day. He will safely return to public engagement and presidential travel," presidential physician Kevin O'Connor said in a letter released by the White House.

Biden had also tested negative earlier on Saturday, though O'Connor said that "in an abundance of caution," the president would remain isolated pending a second negative test.

The 79-year-old president first tested positive for the virus on July 21, and then tested positive for a rebound case of COVID-19 last Saturday and resumed isolation.