WASHINGTON - US President Donald Trump moved to a military hospital for treatment on Friday after being diagnosed with COVID-19 as his administration and re-election campaign scrambled to adjust to an extraordinary twist in his turbulent presidency.

Roughly 17 hours after he announced he had tested positive for the coronavirus, Trump walked slowly from the White House to a waiting helicopter to be taken to Walter Reed National Military Medical Center in Bethesda, Maryland. He wore a mask and business suit and did not speak to reporters.

"I think I'm doing very well, but we're going to make sure that things work out," Trump said in a brief video posted to Twitter.

Trump will work in a special suite at the hospital for the next few days as a precautionary measure, White House press secretary Kayleigh McEnany said.

Trump, 74, has a mild fever, according to a source familiar with the matter. White House doctor Sean P. Conley said he is being treated with an experimental drug cocktail and is "fatigued but in good spirits."

It was the latest setback for the Republican president, who is trailing Democratic rival Joe Biden in opinion polls ahead of the Nov. 3 presidential election.

Trump, who has played down the threat of the coronavirus pandemic from the outset, wrote on Twitter earlier on Friday that he and his wife Melania were going into quarantine after testing positive for the virus, which has killed more than 200,000 Americans and severely damaged the US economy.

An active Twitter user, Trump has posted only the short video since then.

Trump is at high risk because of his age and weight. He has remained in apparent good health during his time in office but is not known to exercise regularly or to follow a healthy diet.

Conley said Trump has received an experimental treatment, Regeneron's REGN-COV2. The drug is one of several experimental COVID-19 drugs known as monoclonal antibodies, which are used for treating a wide range of illnesses. US infectious disease chief Dr. Anthony Fauci is among those saying the technology has promise.

Trump is also taking zinc, Vitamin D, famotidine, melatonin and a daily aspirin.

Stocks on Wall Street closed lower as news of Trump's diagnosis added to mounting uncertainties surrounding the election.

The president's illness upended his re-election campaign with just 31 days to go until Election Day. The campaign said it would postpone rallies and other events where he was scheduled to appear, or take them online.

Biden pulled ads attacking Trump off the air but otherwise continued his campaign, traveling to Michigan on Friday after testing negative for the virus.

At a union hall in Grand Rapids, Biden said he was praying for his rival's recovery. However, he also implicitly criticized Trump, who has mocked Biden for routinely wearing a mask and has held huge campaign rallies with little social distancing.

"Be patriotic," Biden said. "It's not about being a tough guy. It's about doing your part.