U.S. President Donald Trump on Monday returned to the White House after spending three nights in hospital to undergo treatment for a coronavirus infection, with his medical team approving his discharge while cautioning that he may not be "entirely out of the woods yet."

The 74-year-old president tweeted that he is feeling "really good" before leaving Walter Reed National Military Medical Center near Washington in the evening. He said he believes there is little need to fear the virus that has killed some 210,000 of his fellow Americans.

U.S. President Donald Trump removes his mask upon return to the White House from Walter Reed National Military Medical Center on Oct. 5, 2020, in Washington. (Getty/Kyodo)

"Don't be afraid of COVID. Don't let it dominate your life. We have developed, under the Trump Administration, some really great drugs and knowledge. I feel better than I did 20 years ago!" he posted.

With the presidential election looming on Nov. 3, Trump appears to be seeking to project strength by demonstrating that he was able to overcome the respiratory illness quickly.

Trump, wearing a suit, was seen walking out of the hospital and giving a thumbs-up before boarding a helicopter that transported him to the White House. Upon arrival, he walked up the stairs to a balcony, removed his mask and posed while saluting as the helicopter lifted off.

On Sunday, he took a short motorcade ride outside the hospital to wave to gathered supporters, an act that sparked criticism for risking the health of Secret Service agents who accompanied him for what commentators called nothing more than "political theater."

Democratic presidential nominee Joe Biden has extended his lead over the Republican president by 14 percentage points in a new poll by The Wall Street Journal and NBC News. The survey was conducted among registered voters in the wake of a chaotic presidential debate last Tuesday but before news emerged that Trump had tested positive for COVID-19.

Trump tweeted Monday, "Will be back on the Campaign Trail soon!!! The Fake News only shows the Fake Polls."

The president's physician Sean Conley told a joint press conference with other members of the medical team on Monday afternoon that the president has met or exceeded all hospital discharge criteria and that more than 72 hours have passed since he last had a fever.

"Though he may not entirely be out of the woods yet, the team and I agree that all our evaluations, and most importantly, his clinical status, support the president's safe return home, where he'll be surrounded by world-class medical care 24/7," Conley said.

Asked about concerns of a potential worsening or reversal of the president's condition, Conley said the team will remain vigilant through the weekend.

"We're in a bit of uncharted territory when it comes to a patient that received the therapies he has so early in the course," the doctor said. "So we're looking to this weekend. If we can get through to Monday with him remaining the same or improving, better yet, then we will all take that final deep sigh of relief."

The president, whose gender, advanced age and weight put him in a high-risk group, has received an experimental antibody cocktail manufactured by Regeneron Pharmaceuticals Inc., a U.S. biotechnology company.

He has also undergone a five-day course of anti-viral drug remdesivir from Friday and has taken dexamethasone, a steroid, which, according to the World Health Organization, has been shown to be beneficial for critically ill COVID-19 patients.

After Trump's positive diagnosis, more White House officials tested positive for the coronavirus.

Press Secretary Kayleigh McEnany said she tested positive Monday morning, after "testing negative consistently, including every day since Thursday."

She said she was not experiencing any symptoms and would begin a quarantine process.

Hope Hicks, a counselor to the president, tested positive Thursday, which was followed by an announcement by Trump in the early hours of Friday that he and his wife Melania were also infected.

Some Republican senators and other guests who attended a White House event on Sept. 26 for Trump's announcement of his Supreme Court justice nominee have also reportedly tested positive for the virus, raising speculation that it could have been a "super spreader" event.

Over the past months, Trump has been widely criticized for repeatedly downplaying the threat of the virus and for his reluctance to wear or encourage the use of masks to reduce the spread of the disease.