US President Donald Trump has left the military hospital where he is being treated for COVID-19 for a drive-by greeting to supporters who have kept a vigil outside the hospital since his admission three days ago.

Posting on Twitter on Monday morning (Australian time), Mr Trump, 74, announced he was leaving Walter Reed National Military Medical Centre for a “little surprise visit” outside.

“It’s been a very interesting journey,” Mr Trump said.

Currently being treated for COVID-19, Mr Trump sits alongside Secret Service agents inside an SUV. Photo: AAP

“They’ve been out there for a long time, they’ve got Trump flags, and they love our country, so I’m not telling anybody but you but I’m about to make a little surprise visit.”

Minutes later, Mr Trump was seen waving from the back seat of an SUV – wearing a dark suit and a mask and surrounded by Secret Service agents – as the presidential motorcade drove down the street outside the hospital.

The drive-by stunt has drawn criticism, including from one Walter Reed hospital doctor, who said “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. They may die. For political theater. Commanded by Trump to put their lives at risk for theater. This is insanity,” Dr James Phillips posted on Twitter.

“That Presidential SUV is not only bulletproof, but hermetically sealed against chemical attack. The risk of COVID19 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,” the emergency medicine chief and Walter Reed attending doctor wrote.

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The fleet drove past the crowd of Trump supporters before doing a U-turn and turning back into the hospital grounds.

Seven months into the pandemic crisis and with more than 200,000 Americans dead, Mr Trump said in his video that he had learned a lot about COVID-19 during his stay at the military hospital.

“I learned it by really going to school, this is the real school, this isn’t the ‘Let’s read the book school’ and I get it, and I understand it,” he said.

In response, Dr Phillips questioned whether Mr Trump had really learned anything about the virus that has swept through the Republican leadership.

Supporters cheer and wave US and Trump flags as motorcade drives past during a one-minute appearance. Photo: AAP

Mr Trump’s condition improved over the weekend. He has been treated with supplemental oxygen twice during his battle with the lung disease.

His medical team have also given him dexamethasone, which has been shown in studies to improve survival for patients hospitalised with critical COVID-19 who need extra oxygen.

But it should not be given in mild cases as it can limit the body’s own ability to combat the virus, according to guidelines from the Infectious Disease Society of America.

“The fact of the matter is that he’s doing really well,” Dr Sean Conley said on Sunday (local time) outside the Walter Reed centre, where Mr Trump has been receiving treatment since Friday.

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Doctors said Mr Trump had not run a fever since Friday and his liver and kidney function remained normal after the second dose in a five-day course of Remdesivir.

The intravenous antiviral drug has been shown to shorten hospital stays.

Asked what tests had revealed about the condition of Mr Trump’s lungs, Dr Conley said there were “some expected findings, but nothing of any major clinical concern”.

Dr Brian Garibaldi said Mr Trump was given dexamethasone in response to “transient low oxygen levels”.

“He received his first dose of that yesterday and our plan is to continue that for the time being,” Dr Garibaldi said.

Mr Trump is also being given an experimental treatment, Regeneron’s REGN-COV2, as well as zinc, vitamin D, famotidine, melatonin and aspirin.

“Today he feels well, he’s been up and around. Our plan for today is to have him eat and drink, be up out of bed as much as possible, to be mobile,” Dr Garibaldi said.

“If he continues to look and feel as well as he does today, our hope is that we can plan for a discharge as early as tomorrow to the White House where he can continue his treatment course.”

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Mr Trump released a four-minute video on Saturday in which he said the “real test” of his condition will come in the next few days.

“I guess that’s the real test, so we’ll be seeing what happens over those next couple of days,” he said into the camera, looking tired and wearing a jacket and open-necked shirt.

Differing assessments of Mr Trump’s health from administration officials on Saturday left it unclear how ill the President had become since he  revealed on Thursday night (local time) that he had tested positive for the coronavirus.

A White House team of doctors said on Saturday that Mr Trump’s condition was improving and that he was already talking about returning to the White House.

Within minutes, White House chief of staff Mark Meadows gave a less rosy assessment.

“The President’s vitals over the last 24 hours were very concerning and the next 48 hours will be critical in terms of his care. We’re still not on a clear path to a full recovery,” he said.

-with AAP