US President Donald Trump staged a dramatic return to the White House after leaving the military hospital where he was receiving an unprecedented level of care for Covid-19.

He immediately ignited a new controversy by declaring that despite his illness the nation should not fear the virus that has killed more than 210,000 Americans and then he entered the White House without a protective mask.

“If the President bounces back onto the campaign trail, he will be an invincible hero, who not only survived every dirty trick the Democrats threw at him, but the Chinese virus as well,” he wrote on Twitter.

Trump's message alarmed infectious disease experts and suggested the president's own illness had not caused him to rethink his often-cavalier attitude toward the disease, which has also infected the first lady and several White House aides, including new cases revealed on Monday.

Landing on Monday night at the White House on Marine One, Trump gingerly climbed the South Portico steps, removed his mask and declared: "I feel good".

He gave a double thumbs-up to the departing helicopter from the portico terrace, where aides had arranged American flags for the sunset occasion. He entered the White House, where aides were visible milling about the Blue Room, without wearing a face covering.

The president left Walter Reed National Military Medical Centre, where his doctor, Navy Cmdr Sean Conley, said earlier on Monday that the president remains contagious and would not be fully out of the woods for another week but that Trump had met or exceeded standards for discharge from the hospital.

Trump is expected to continue his recovery at the White House, where the reach of the outbreak that has infected the highest levels of the US government is still being uncovered.

Still, just a month before the election and anxious to project strength, Trump tweeted before leaving the hospital, "Will be back on the campaign trail soon!!!"

And in case anyone missed his don't-worry message earlier, he rushed out a new video from the White House.

"Don't be afraid of it,” Trump said of the virus. “You're going to beat it. We have the best medical equipment, we have the best medicines." His remarks were strong, but he was taking deeper breaths than usual as he delivered them.

Trump's nonchalant message about not fearing the virus comes as his own administration has encouraged Americans to be very careful and take precautions to avoid contracting and spreading the disease as cases continue to spike across the country.

For more than eight months, Trump's efforts to play down the threat of the virus in hopes of propping up the economy ahead of the election have drawn bipartisan criticism.

"We have to be realistic in this: Covid is a complete threat to the American population," Dr David Nace of the University of Pittsburgh Medical Centre, said of Trump's comment.

"Most of the people aren't so lucky as the president, with an in-house medical unit and access to experimental treatments," added Nace, an expert on infections in older adults.

"It's an unconscionable message," agreed Dr Sadiya Khan of Northwestern University Feinberg School of Medicine. "I would go so far as to say that it may precipitate or worsen spread."

Likewise, Democratic presidential nominee Joe Biden, who spent more than 90 minutes on the debate stage with Trump last week, said during an NBC town hall on Monday night that he was glad Trump seemed to be recovering well, “but there's a lot to be concerned about — 210,000 people have died".

"I hope no one walks away with the message that its not a problem." Biden tested negative for the virus on Sunday.

There was pushback from a prominent Trump political supporter as well.

Republican Senator John Cornyn of Texas told the Houston Chronicle editorial board that Trump had let his guard down in his effort to show that the country was moving beyond the virus and had created confusion about how to stay safe.

Conley said that because of Trump's unusual level of treatment so early after discovery of his illness he was in uncharted territory. But the doctor also was upbeat at an afternoon briefing and said the president could resume his normal schedule once there is no evidence of live virus still present.

According to the Centers for Disease Control and Prevention, those with mild to moderate symptoms of Covid-19 can be contagious for as many as — and should isolate for at least — 10 days.

Trump’s arrival back at the White House raised new questions about how the administration was going to protect other officials from a disease that remains rampant in the president’s body. Press secretary Kayleigh McEnany announced she had tested positive for the virus on Monday morning and was entering quarantine.

There were also lingering questions about potential long-term effects to the president — and even when he first came down with the virus.

Conley repeatedly declined to share results of medical scans of Trump’s lungs, saying he was not at liberty to discuss the information because Trump did not waive doctor-patient confidentiality on the subject.

Covid-19 has been known to cause significant damage to the lungs of some patients. Conley also declined to share the date of Trump’s most recent negative test for the virus — a critical point for contact tracing and understanding where Trump was in the course of the disease.

Only a day earlier, Trump suggested he had finally grasped the true nature of the virus, saying in a video, “I get it.”

But on Sunday afternoon, he ventured out of the hospital while contagious to salute cheering supporters by motorcade — an outing that disregarded precautions meant to contain the virus.

At the hospital, doctors revealed that his blood oxygen level had dropped suddenly twice in recent days and that they gave him a steroid typically only recommended for the very sick.

Trump’s experience with the disease has been dramatically different from most Americans, who do not have access to the same kind of monitoring and care.

While most must cope with their symptoms — and fear of whether they’ll take a turn for the worse — at home and alone, Trump has been staying in the presidential suite of one of the nation’s best hospitals and has been given experimental drugs not readily available to the public. He returns to the White House, where there is a team of doctors on call with 24-hour monitoring.