US President Donald Trump has 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.

Trump, confined to the White House where he is being treated, yesterday said he does not believe he is contagious and is feeling good enough to resume campaign rallies as he tries to boost his flagging reelection bid.

"I'm back because I'm a perfect physical specimen," Trump told Fox Business Network's Maria Bartiromo in his first interview since it was revealed nearly a week ago that he had tested positive for the coronavirus.

Trump's contention he was no longer contagious was not yet backed up by solid evidence from his doctors. The White House has refused to say when his last negative test for the virus was and, as a result, it is unclear how long he has been positive.

The Republican president is under pressure to actively campaign again to try to inject new energy into his reelection bid as he faces a deficit in opinion polls in battleground states with Election Day now less than four weeks away.

"I'm feeling good, really good," said Trump, who spent three nights being treated at a military hospital outside Washington until his discharge on Monday. Trump said he had stopped taking "most therapeutics" for the virus but was still on steroids.