US President Donald Trump on Sunday briefly left Walter Reed National Military Medical Center, where he has been since Friday after testing positive for the novel coronavirus, and took a ride in an SUV to wave to supporters.

Trump wore a mask as he sat in the rear passenger seat of a black Chevrolet Suburban minutes after tweeting a video promising a "little surprise visit" to the crowd.

"It's been a very interesting journey," Trump says in the video. "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 book' school. I get it. I understand it, and it's a very interesting thing, and I'm going to be letting you know about it."

A doctor on Trump's medical team said Sunday that the president could be discharged as soon as Monday.

"If he continues to look and feel as well as he does today, our hope is to plan for a discharge as early as tomorrow to the White House, where he can continue his treatment course," Dr Brian Garibaldi said outside the medical center in Bethesda, Maryland.

Dr Sean Conley, the White House physician, said Sunday that the president "continues to improve" and "has remained without fever since Friday morning".

"His vital signs are stable. From a pulmonary standpoint, he remains on room air this morning and is not complaining of shortness of breath or other significant respiratory symptoms," Conley said.

Trump's condition stirred some controversy over the weekend with the doctors' upbeat prognosis and comments by White House Chief of Staff Mark Meadows conflicting, with Meadows saying that Trump's condition was "very concerning".

Conley offered an explanation to reporters Sunday.

"Thursday night into Friday morning when I left the bedside, the president was doing well with only mild symptoms and his oxygen was in the high 90s," he said. "Friday morning when I returned to the bedside, the president had a high fever and his oxygen saturation was transiently dipping below 94 percent."

Conley said he became "concerned for possible rapid progression of the illness" and administered the steroid dexamethasone, which is not normally used in mild cases of COVID-19, according to The New York Times.

The World Health Organization last month recommended that dexamethasone be given to patients only with "severe and critical" COVID-19.

"I recommended to the president that we try supplemental oxygen and see how he'd respond. He was fairly adamant he didn't need it. He was not short of breath. He was tired, had a fever, that was about it," Conley said.

"After about a minute — on only 2 liters — his saturation levels were back over 95 percent. He stayed on that for about an hour maybe. … Later that day by the time the team here was at the bedside, the president had been up out of bed moving about the residence with only mild symptoms," he said.

Trump, 74, also is receiving the antiviral drug Remdesivir and a cocktail of antibodies.

So far, there doesn't seem to be too much of an impact on the presidential campaign, with the Nov 3 election less than a month away, although Trump's SUV jaunt could reduce reservations Democrats may have about criticizing him on the campaign trail.

Trump senior adviser Jason Miller said Sunday on NBC's Meet the Press that there was no interest in holding remote debates with Democratic presidential candidate Joe Biden. The two remaining debates are scheduled for Oct 15 and Oct 22.

Kate Bedingfield, a deputy campaign manager for Biden, said on ABC's This Week on Sunday that the former vice-president, 77, was open to the debates continuing as scheduled, as long as proper health precautions are taken, and Trump's health permits.

"We send President Trump our best. We hope that he is well and able to debate," Bedingfield said Sunday. "If he is, Joe Biden will certainly be there."

A Reuters/Ipsos poll published Sunday found that Biden had a 10-point lead over Trump nationally, a slight rise from where it has been for two months.

Biden tweeted Sunday: "Today marks 30 days until Election Day. 30 days until we begin to restore the soul of the nation."

Vice-President Mike Pence, who again tested negative for the virus on Sunday, will face US Senator Kamala Harris, Biden's running mate, in a debate Wednesday in Salt Lake City, Utah. They now will be spaced 12 feet apart instead of the initially agreed upon 7 feet, for health precautions.

"He will be hitting the trail," Miller told CNN of Pence, with some observers questioning the risk. "And he's going to have a very full, aggressive schedule."

The New Jersey Department of Health said Sunday that the White House had given it a list of more than 200 people who attended a fundraiser for Trump at his golf club in Bedminster on Thursday, after the White House had learned that adviser Hope Hicks had tested positive for the virus.

Conley offered an explanation to reporters Sunday.

"Thursday night into Friday morning when I left the bedside, the president was doing well with only mild symptoms and his oxygen was in the high 90s," he said. "Friday morning when I returned to the bedside, the president had a high fever and his oxygen saturation was transiently dipping below 94 percent."

Conley said he became "concerned for possible rapid progression of the illness" and administered the steroid dexamethasone, which is not normally used in mild cases of COVID-19, according to The New York Times.

The World Health Organization last month recommended that dexamethasone be given to patients only with "severe and critical" COVID-19.

"I recommended to the president that we try supplemental oxygen and see how he'd respond. He was fairly adamant he didn't need it. He was not short of breath. He was tired, had a fever, that was about it," Conley said.

"After about a minute — on only 2 liters — his saturation levels were back over 95 percent. He stayed on that for about an hour maybe. … Later that day by the time the team here was at the bedside, the president had been up out of bed moving about the residence with only mild symptoms," he said.

Trump, 74, also is receiving the antiviral drug Remdesivir and a cocktail of antibodies.

So far, there doesn't seem to be too much of an impact on the presidential campaign, with the Nov 3 election less than a month away, although Trump's SUV jaunt could reduce reservations Democrats may have about criticizing him on the campaign trail.

Trump senior adviser Jason Miller said Sunday on NBC's Meet the Press that there was no interest in holding remote debates with Democratic presidential candidate Joe Biden. The two remaining debates are scheduled for Oct 15 and Oct 22.

Kate Bedingfield, a deputy campaign manager for Biden, said on ABC's This Week on Sunday that the former vice-president, 77, was open to the debates continuing as scheduled, as long as proper health precautions are taken, and Trump's health permits.

"We send President Trump our best. We hope that he is well and able to debate," Bedingfield said Sunday. "If he is, Joe Biden will certainly be there."

A Reuters/Ipsos poll published Sunday found that Biden had a 10-point lead over Trump nationally, a slight rise from where his lead has been for two months.

Biden tweeted Sunday: "Today marks 30 days until Election Day. 30 days until we begin to restore the soul of the nation."

Vice-President Mike Pence, who again tested negative for the virus on Sunday, will face US Senator Kamala Harris of California, Biden's running mate, in a debate Wednesday in Salt Lake City, Utah. They now will be spaced 12 feet apart instead of the initially agreed upon 7 feet, for health precautions.

"He will be hitting the trail," Miller told CNN of Pence, with some observers questioning the risk. "And he's going to have a very full, aggressive schedule."

The New Jersey Department of Health said Sunday that the White House had given it a list of more than 200 people who attended a fundraiser for Trump at his golf club in Bedminster on Thursday, after the White House had learned that adviser Hope Hicks had tested positive for the virus.