Donald Trump has left the Walter Reed military hospital where he is being treated for COVID-19 to thank supporters gathered outside.

It came immediately after the US President posted an upbeat video message thanking his medical team, saying he "learned a lot" about the coronavirus.

"We're getting great reports from the doctors, this is an incredible hospital Walter Reed, the work they do is just absolutely amazing and I want to thank them all, the nurses, the doctors, everybody here," he said.

"It's been a very interesting journey. I learned a lot about COVID. I learned it by really going to school - this is a real school, this isn't the 'let's read the book' school, and I get it, I understand it, and it's a very interesting thing, and I'm going to be letting you know about it."

He thanked the crowd gathered outside waving flags and signs, saying he was going to pay them "a little surprise visit".

Mr Trump was then seen waving at supporters as his motorcade drove past.

He has since returned to the hospital in Bethesda, Maryland, where he was flown late Friday after his symptoms worsened.

"President Trump took a short, last-minute motorcade ride to wave to his supporters outside and has now returned to the Presidential Suite inside Walter Reed," the White House said in a statement.

The brief excursion came after Mr Trump's doctors said he had "continued to improve" and could be discharged as early as Monday, even as they admitted his symptoms had been worse than they previously reported.

Critics slammed the President for putting lives at risk - not of his supporters, but the Secret Service agents in the car with him.

"That Presidential SUV is not only bulletproof, but hermetically sealed against chemical attack," tweeted Dr James Phillips, an attending physician at Walter Reed.

"The risk of COVID-19 transmission inside is as high as it gets outside of medical procedures. The irresponsibility is astounding. My thoughts are with the Secret Service forced to play."

Dr Phillips noted that "every single person in the vehicle during that completely unnecessary Presidential 'drive-by' just now has to be quarantined for 14 days". "They might get sick," he said. "They may die. For political theatre. Commanded by Trump to put their lives at risk for theatre. This is insanity."