The move has disregarded precautions meant to contain the deadly virus which has forced his hospitalisation and killed more than 209,000 Americans, reports AP.

His surprise public appearance in a drive-past to greet supporters is drawing criticisms, reports BBC.

"More than 205,000 Americans are dead. We need leadership. Not photo ops," tweeted Hakeem Jeffries, chairman of the Democrats in the US House of Representatives.

Wearing a mask, the president waved to well-wishers from behind the glass of a sealed car after tweeting that he would leave Walter Reed hospital to pay a "surprise visit" to "patriots" outside.

Hours earlier, Trump’s medical team reported that his blood oxygen level dropped suddenly twice in recent days and that they gave him a steroid typically only recommended for the very sick.

Still, the doctors said, Trump’s health is improving and that he could be discharged as early as Monday.

Earlier, it emerged that his condition was more serious than previously reported when he went to hospital Friday evening.

The White House had said Trump was experiencing "mild symptoms" of Covid-19, but it was confirmed at the weekend that he received supplemental oxygen after his levels dipped twice within two days.

He was given the steroid dexamethasone, which is normally reserved for serious cases, according to experts.

However, Trump sought to project an image of strength, tweeting images of himself appearing to work from the hospital and a video message teasing his surprise drive past.

In a tweet, the president - dressed in a suit jacket and shirt with no tie - said: "I learned a lot about Covid. I learned it by really going to school. This is the real school. This isn't the let's read the books school. I get it and I understand it. It's a very interesting thing, I'll be letting you know about it."

The president's diagnosis, which he made public in a tweet early on Friday, has upended his election campaign.

 “This is insanity,” Dr. James P. Phillips, an attending physician at Walter Reed who is a critic of Trump and his handling of the pandemic.

“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,” the doctor added. “Commanded by Trump to put their lives at risk for theater.”

His doctors sidestepped questions on Sunday about exactly when Trump’s blood oxygen dropped — an episode they neglected to mention in multiple statements the day before — or whether lung scans showed any damage.

It was the second straight day of obfuscation from a White House already suffering from a credibility crisis.

And it raised more doubts about whether the doctors treating the president were sharing accurate, timely information with the American public about the severity of his condition.

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