"Tonight, FLOTUS and I tested positive for COVID-19. We will begin our quarantine and recovery process immediately. We will get through this TOGETHER!," he tweeted.

Mrs Trump said the couple were "feeling good" and had postponed up coming engagements.

Mr Trump is 74 years old, putting him at higher risk of serious complications from a virus that has now killed more than 200,000 people in the United States. Most people develop only mild symptoms. But some people, usually those with other medical complications, develop more severe symptoms, including pneumonia, which can be fatal.

His health scare, which rattled financial markets, shifts the 2020 presidential campaign dramatically.

With only weeks before the November 3 election, Mr Trump will now be sidelined from doing in-person events after holding a slew of rallies with thousands of people attending, against the advice of public health professionals.

Mr Trump's positive test also means that others at the highest levels of the US government have been exposed and may have to quarantine, too.

A White House official said early on Friday that contact tracing was under way.

The President's physician, Sean Conley, confirmed the diagnosis in a statement.

"The President and First Lady are both well at this time, and they plan to remain at home within the White House during the convalescence," he wrote.

"The White House medical team and I will maintain a vigilant watch, and I appreciate the support provided by some of our country's greatest medical professionals and institutions. Rest assured, I expect the President to continue carrying out his duties without disruption while recovering, and I will keep you updated on any future developments."

The diagnosis marks a major blow for a president who has been trying desperately to convince the American public that the worst of the pandemic is behind them even as cases continue to rise with less than four months before Election Day.

And it stands as the most serious known public health scare encountered by any sitting American president in recent history.

The president rarely wears a mask and has ridiculed others, including Democratic presidential nominee Joe Biden, for doing so regularly. Biden has criticised Trump sharply for his response to the pandemic, while the president has touted his own management of it.

The White House issued a new schedule for Trump's activities on Friday that did not include his planned trip to Florida. Biden is scheduled to campaign in Michigan on Friday. Both states are swing states that could decide the US election.

Trump, who has been criticised for questioning the efficacy of wearing a face covering, produced a mask from his pocket in the first presidential debate on Tuesday and said, "I wear masks when needed. When needed, I wear masks."

He then ridiculed Biden for wearing them regularly: "I don't wear masks like him. Every time you see him, he's got a mask. He could be speaking 200 feet away from them and he shows up with the biggest mask I've ever seen."

Trump has taken pride in his big campaign rallies, with crowds of people who do not wear masks or maintain social distance.

Mr Trump is the second leader of one of Australia's main allies to contract the virus. UK Prime Minister Boris Johnson spent three nights in intensive care in April.

Canadian Prime Minister Justin Trudeau went into quarantine in March after his wife was diagnosed with the coronavirus, while Brazil's President Jair Bolsonaro called it a "little flu" after being infected in July.

Australian cabinet minister David Littleproud expressed Australia's best wishes for a speedy recovery.

"It demonstrates that no one is immune from catching COVID-19," he told the ABC.

"No matter the precautions, we are all susceptible to this and it is important that we understand that as a global community."

The 74-year-old US president is in the middle of an election race against 77-year-old Democratic candidate Joe Biden, who he debated face-to-face earlier in the week .

Mr Trump's positive test came just hours after the White House announced that senior aide Hope Hicks came down with the virus after traveling with the president several times this week.

Ms Hicks began feeling mild symptoms during the plane ride home from a rally in Minnesota Wednesday evening, according to an administration official, who spoke on condition of anonymity to disclose private information.

She was quarantined away from others on the plane and her diagnosis was confirmed Thursday, the person said.

Ms Hicks, who serves as counsellor to Mr Trump, also travelled with him to the first presidential debate in Cleveland on Tuesday. She has been the most senior White House official to test positive at that point.

Mr Trump travelled to New Jersey on Thursday for a fundraiser and the White House sent out a schedule for Friday that includes an intelligence briefing, a phone call on COVID-19 support for "vulnerable seniors," a fundraiser at his Washington DC hotel and another rally in Sanford, Florida.

READ MORE:

In a statement before Mr Trump announced he had contracted the virus, White House spokesman Judd Deere said the president "takes the health and safety of himself and everyone who works in support of him and the American people very seriously.".

"White House Operations collaborates with the Physician to the President and the White House Military Office to ensure all plans and procedures incorporate current CDC guidance and best practices for limiting COVID-19 exposure to the greatest extent possible both on complex and when the President is travelling," Mr Deere said.

Ms Hicks travelled with the president multiple times this week, including aboard Marine One, the presidential helicopter, and on Air Force One to a rally in Minnesota Wednesday, and aboard Air Force One to Tuesday night's first presidential debate in Cleveland.

Multiple White House staffers have tested positive for the virus, including Katie Miller, Vice President Mike Pence's press secretary, national security adviser Robert O'Brien, and one of the president's personal valets. Kimberly Guilfoyle, who is dating Mr Trump's oldest son, Donald Trump Jr., tested positive in South Dakota before an Independence Day fireworks show at Mount Rushmore.

Still, Mr Trump has consistently played down concerns about being personally vulnerable to contracting COVID-19.

"I felt no vulnerability whatsoever," he told reporters back in May.

ABC host Dr Norman Swan said at age 74 a man had a mortality rate of between five per cent and 12 per cent from coronavirus.

However, because Mr Trump is considered by the presidential physician to be obese, the range would be between seven per cent and 18 per cent.

Futures for the S&P 500 fell 1.8 per cent in Asian trading a fter the news, extending earlier losses, while barometers of risk sentiment, such as the Australian dollar and Treasury yields, dipped.

AP