Joe Biden, the oldest person ever to serve as president of the United States, has tested positive for COVID-19, is experiencing mild symptoms and will continue working but in isolation, the White House says.

Biden, 79, had a runny nose, fatigue and an occasional dry cough, symptoms which he began to experience late on Wednesday, White House physician Kevin O’Connor said in a note released on Thursday.

Biden had begun taking the antiviral treatment Paxlovid, which has been shown to reduce the risk of severe disease by nearly 90 per cent in high-risk patients if given within the first five days of infection.

Fully vaccinated and twice boosted, Biden said he was “doing well” in a video posted on his Twitter account. 

He also said he was “getting a lot of work done” and would continue with his duties.

A photograph on his Twitter account showed him smiling and sitting at a desk with papers.

White House COVID-19 co-ordinator, Dr Ashish Jha, said Biden’s oxygen levels were normal and he would isolate for five days and return to public events once he had a negative test.

Biden became ill at a time when his administration is grappling with soaring inflation and Russia’s land assault on Ukraine.

His popularity has dropped sharply in the past year. A Reuters/Ipsos survey this week showed 36 per cent of people in the US approve of his job performance.

His illness forced cancellation of a trip to Pennsylvania where Biden intended to lay out plans to ask Congress for $US37 billion ($A54 billion) for crime prevention programs.

At Biden’s last physical in November 2021, doctors reported the president had atrial fibrillation, a common irregular heartbeat for which he takes Eliquis, a drug designed to prevent blood clots and reduce the risk of heart attacks and stroke.

Jha said Biden would stop taking Eliquis and the statin Crestor while on his Paxlovid treatment to avoid a negative interaction between the drugs.

Multiple members of Biden’s administration and other senior figures in Washington DC have tested positive for the coronavirus in recent months, including Vice-President Kamala Harris and House Speaker Nancy Pelosi, both of whom have since tested negative and resumed working.

Biden set up strict COVID-19 safety protocols at the White House, urged the public to take the virus seriously and campaigned for everyone to get fully vaccinated.

He is tested regularly and anyone who meets him or travels with him is tested beforehand, the White House has said. 

Biden had last tested negative on Tuesday.

He has stopped wearing a mask at public events in recent months, and the White House dropped its mask requirement before his March 1 State of the Union Address.

Administration officials have acknowledged that Biden’s regular contact with advisers and supporters could expose him to COVID-19.

Biden’s wife Jill tested negative on Thursday in Detroit and plans to maintain her full schedule of travel in Michigan and Georgia, her office says.