US President Donald Trump was hospitalized and given an experimental Covid-19 treatment, but said he was "well," following bombshell news that the Republican had contracted the virus, knocking him off the campaign trail a month from the US election.

After having not been seen in public since his shock announcement that he and first lady Melania Trump tested positive for the coronavirus, the president walked out of the White House Friday evening, wearing a mask, and flew by helicopter to the Walter Reed military hospital outside Washington.

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In an 18-second video recorded inside the White House and released on Twitter, Trump broke his silence, saying he was being hospitalized but "I think I'm doing very well."

"We're going to make sure that things work out," he said, adding that the first lady was also "doing very well."

Press Secretary Kayleigh McEnany said Trump was receiving the anti-viral drug remdesivir following consultation with specialists. The president was "not requiring any supplemental oxygen," she said in a statement late Friday.

TV crews and reporters stand across from the Walter Reed National Military Medical Center in Bethesda, Maryland, where US President Donald Trump arrived after testing positive for Covid-19. Trump will spend the coming days in a military hospital to undergo treatment for the coronavirus.–AFP

Earlier, she said that medical experts recommended Trump work from the presidential offices at Walter Reed "for the next few days."

The hospitalization indicated an intense effort to make sure the president's reportedly "mild" symptoms do not deteriorate.

The development also highlighted the uncharted waters for the US election on November 3, with Trump -- who is well behind his Democratic opponent Joe Biden in the polls -- having to freeze much of his campaign.

At first, aides gave rosy assessments, with Trump's chief of staff Mark Meadows saying the president, 74, had only mild symptoms, was in "good spirits" and feeling "very energetic."

Later Friday, a letter issued by White House physician Sean Conley indicated something more serious.

Conley said Trump received a single dose of Regeneron's antibody cocktail, a treatment undergoing clinical trials but not yet approved by regulators. "He's being evaluated by a team of experts, and together we'll be making recommendations to the President and First Lady in regards to next best steps," Conley wrote.

The president's son Donald Trump Jr told Fox News his father was "obviously taking it very seriously, but he's a fighter."

- Biden out on his own -

Trump's illness upended the White House race, with Biden suddenly finding himself alone on the campaign trail -- and able to argue that his distinctly more cautious approach to Covid-19 had been vindicated.

Biden has made Trump's frequent downplaying of the pandemic and mixed messaging on mask-wearing a central campaign theme, while Trump has tried repeatedly to shift the narrative to areas where he feels stronger, like the economy.

The former vice president, who stood in close proximity to Trump for 90 minutes during their ill-tempered first debate Tuesday in Cleveland, announced that he and his wife Jill tested negative Friday.

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Underlining his sudden advantage in the bitter race, Biden, 77, travelled to Grand Rapids, Michigan, going ahead with a previously scheduled campaign stop.

Biden said he was praying for Trump and his family, and his campaign announced it would take down all negative ads. However, Biden also reminded voters that he has pushed consistently for taking seriously the coronavirus, which has killed more than 208,000 Americans, unlike his opponent who has repeatedly mocked the Democrat for his rigorous use of masks.

"Be patriotic," Biden told supporters in Michigan through a blue surgical mask. "It's not about being a tough guy. It's about doing your part."

- Campaign grounded -

Trump's shock diagnosis was a huge blow to his re-election effort, triggering a freeze on plans to barnstorm the country in an attempt to catch up in the polls.

The Trump campaign said all planned events involving the president were either being postponed or going virtual -- starting with the cancellation of a Florida rally Friday, one Saturday in Wisconsin and others in western states like Arizona next week.

Even the second Trump-Biden debate, scheduled for October 15, is now in some doubt.

Rallies are such a key part of the Trump brand that his sudden inability to travel leaves the campaign scrambling to reinvent its strategy. In addition, Trump has made his argument that coronavirus dangers are overblown a central plank in his re-election platform.

- Contact tracing -

News of Trump's infection came after one of his closest advisors, Hope Hicks, tested positive -- sparking fears of a cluster of cases emanating from the heart of the White House.

Trump met with dozens of people through the week and reportedly went to a fundraiser in New Jersey after it was known that Hicks had contracted the virus.

The White House said it was carrying out contact tracing, while Melania Trump's spokeswoman said the couple's 14-year-old son Barron had tested negative.

Vice President Mike Pence and other senior figures tested negative. The White House said Supreme Court nominee Amy Coney Barrett was negative too.

Former White House top aid Kellyanne Conway, however, announced late Friday she had tested positive with "mild" symptoms.

In his 70s and technically obese, Trump is in a higher-risk category.

Daniel Griffin, an infectious disease specialist in the New York area, told AFP Trump had an estimated 20 percent chance of developing severe disease requiring oxygenation.

As the news rattled global stock markets, leaders including Germany's Angela Merkel, Britain's Boris Johnson, and Russia's Vladimir Putin wished the president and first lady a speedy recovery.

Trump's predecessor Barack Obama noted the US is "in the midst of a big political fight" but set aside the bitter election battle to extend "best wishes" to the president and first lady.