Democratic presidential candidate Joe Biden said Friday he has tested negative for Covid-19, three days after his debate with Donald Trump, who has contracted the illness.

"I'm happy to report that Jill and I have tested negative for Covid," Biden said. "I hope this serves as a reminder: wear a mask, keep social distance, and wash your hands."

Minutes before, his doctor Kevin O'Connor issued a statement via the 77-year-old former vice president's campaign about the negative result.

"Vice President Joe Biden and Dr Jill Biden underwent PCR testing for Covid-19 today and Covid-19 was not detected," O'Connor said.

Trump's positive diagnosis - announced in the early hours of Friday after one of his senior aides, Hope Hicks, tested positive - has jolted the race for the White House with a month to go.

Earlier, Biden had wished his 74-year-old adversary and his wife Melania a swift recovery from Covid-19.

"Jill and I send our thoughts to President Trump and First Lady Melania Trump for a swift recovery," Biden tweeted.

"We will continue to pray for the health and safety of the president and his family."

Biden's running mate Kamala Harris put out a similar tweet, saying she and her husband Doug are "keeping them and the entire Trump family in our thoughts."

Vice President Mike Pence has tested negative, his spokesman said Friday.

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