US President Donald Trump on Wednesday declared his Covid-19 illness a "blessing from God," as he got back to work in the Oval Office despite warnings that his return could put others at risk.

Trump has worked hard since he checked out of the hospital on Monday -- including a made-for-television return to the White House -- to cast his illness as a personal triumph.

"I think this is a blessing from God that I caught it," Trump said in a video, referring to the virus which has killed over 210,000 people in the United States.

But Democrat Joe Biden, who leads Trump in the latest polls ahead of the November 3 vote, attacked the president's downplaying of the risk.

"I think it's a tragedy that the president deals with Covid like it is something not to worry about when over 210,000 people (in the US) have died," Biden told reporters.

The behavior of the president -- who has faced fierce criticism for his handling of the outbreak -- is under scrutiny as the number of positive cases climbs among people working at the White House and elsewhere in Washington.

On Wednesday the Pentagon confirmed that one of the Marine Corps' top officers, General Gary Thomas, tested positive for the virus.

In his emblematic red tie, Trump attributed his recovery since being diagnosed Thursday and hospitalized Friday to an antibody cocktail, an experimental treatment that has not even received an emergency use authorization.

"I want everybody to be given the same treatment as your president, because I feel great," he said.

After contradictory statements whether the president was back at work on Tuesday, a senior administration official told AFP that Trump was present Wednesday with an "extremely limited" number of staff in his office.

"Was just briefed on Hurricane Delta," the president tweeted Wednesday, seemingly eager to project his return to work.

Just after stepping off a helicopter upon his return to the White House, Trump climbed the stairs of his residence and pulled off his face mask -- a tool against the virus that he has frequently minimized.

"Don't let (Covid-19) dominate you. Don't be afraid of it," he urged Americans in his homecoming speech, despite the US having the largest total number of coronavirus infections and deaths in the world.

Trump arrived by an outside door to avoid passing through the crowded corridors of his residence, the senior administration official told AFP.

- 'Wacko for years' -
By Wednesday, doctors reported the president had been free of Covid-19 symptoms for 24 hours and has not had a fever in four days.

"The president this morning says 'I feel great,'" doctor Sean Conley said in a brief update. 

In an apparent attempt to provide greater clarity over how the disease has progressed, the update also detailed that Covid antibodies were found in Trump's blood tests on Monday.

Florian Krammer, professor at Icahn School of Medicine in New York, told AFP those results don't mean much at this stage.

"It could well be that the majority of what is detected came from the transfusion," he said referring to the experimental antibody treatment Trump got Friday. 

Trump has vowed to return to campaigning shortly and to participate in the second presidential debate against Biden in Miami on October 15.

As Trump seeks to revive his reelection campaign in the face of weak polling data just four weeks away from the November 3 election, he labeled Biden "a wacko."

Latest polls forecast a clear victory for Biden, with CNN giving the Democrat a national advantage of 57 percent to 41 percent among likely voters, and women voters going 66 to 32 percent in his favor.

"He's been a wacko for years, and everyone knows it," Trump tweeted. "Notice how all of the bad things, like his very low IQ, are no longer reported? Fake News!"

Biden traded blows with Trump in last week's chaotic debate, but the former vice president has also appealed to a broad-based yearning for calm.

Meanwhile, Trump spoke on Wednesday by phone with British premier Boris Johnson, who went into intensive care with the virus earlier this year.

"Very thankful for his friendship and support as I recovered from the China Virus," the Republican billionaire wrote on Twitter, referring to the new coronavirus in a way that has ratcheted up tensions with Beijing.

"I am looking forward to working with him for many years to come, a great guy," added the president, who trails his Democratic opponent Joe Biden by a wide margin in the polls ahead of the November 3 presidential election.

Johnson, who had been slow to act in the UK at the start of the pandemic, had to spend three nights in intensive care in April after he was infected with Covid-19.

Since then, questions about the impact of the disease on his health have been recurring in Britain, where he has also been criticized for his erratic management of the health crisis. 

Trump, who has consistently downplayed the pandemic and has long refused to wear a mask in public, tested positive for Covid-19 last week and was hospitalized from Friday to Monday.  

On Wednesday, he returned to the Oval Office despite the risk of contagion, trying to stage his return to work as he is confined to the White House and deprived of field campaigning with 27 days to go before the election.