President Donald Trump and his doctors over the weekend struck an upbeat tone though it was marred by seemingly conflicting signals about his condition, with Mr Trump even making a surprise outing to greet supporters outside the hospital where he is being treated for Covid-19.

Mr Trump's medical team was still optimistic that he would be able to return to the White House yesterday, and would make the decision later in the day, his chief of staff Mark Meadows told Fox News.

Mr Meadows added that the President's condition had improved overnight and he was ready to get back to a normal working schedule.

Markets yesterday opened higher and rose after Mr Meadows' comments, with the Dow Jones Industrial Average up 370 points, the Nasdaq Composite 170 points higher, and the S&P 500 adding 45 points.

But observers were concerned by White House physician Sean Conley's announcement at a press conference on Sunday that the President had been administered oxygen after two episodes of transient drops in oxygen saturation, something Dr Conley had not mentioned the day before despite being directly asked about it.

Medical experts were also concerned by Dr Conley's news that Mr Trump was being treated with dexamethasone, a steroid meant for severely ill and critical patients, and pointed out that its possible side effects include mood swings.

They were also not reassured by Mr Trump's string of Twitter posts yesterday morning, a series of 18 tweets before 8am urging people to vote for him and his platform.

In one example, he said: "FIGHT THE CORRUPT FAKE NEWS MEDIA. VOTE!"

But the activity was in line with Mr Trump's drive all through the weekend to project a show of strength, culminating in an evening excursion to greet supporters outside the Walter Reed National Military Medical Centre.

"We have enthusiasm like probably nobody's ever had," said Mr Trump in a video message on Twitter, hours after the release of a Reuters-Ipsos poll showing his Democratic rival Joe Biden's lead over him had widened. "We have more enthusiasm than maybe anybody."

He also sought to show he was taking the coronavirus situation seriously and had a handle on the situation, saying: "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 books school. I get it and I understand it."

Wearing a face mask and a suit jacket, the President rode in an armoured SUV, waving to cheering supporters displaying US and Trump campaign flags outside the hospital grounds.

But the outing, at odds with medical experts' advice for Covid-19 positive patients to remain isolated, was criticised as irresponsible and endangering his driver and the Secret Service members who rode with him in the small airtight space.

Dr James Phillips, an attending physician at the Walter Reed medical centre, said that because the presidential vehicle was sealed against chemical attack, "the risk of Covid-19 transmission inside is as high as it gets outside of medical procedures".

"Every single person in the vehicle during that completely unnecessary Presidential 'drive-by' just now has to be quarantined for 14 days. They might get sick. They may die. For political theatre. Commanded by Trump to put their lives at risk for theatre. This is insanity," Dr Phillips, who is chief of disaster medicine at George Washington University, wrote on Twitter.

Meanwhile, First Lady Melania Trump, who had also tested positive for the coronavirus, said she was feeling good and would continue to rest in the White House.

"My family is grateful for all of the prayers and support," she wrote on Twitter yesterday. "Thank you to medical staff and caretakers everywhere, and my continued prayers for those who are ill or have a family member impacted by the virus."

Separately, White House press secretary Kayleigh McEnany said that she had tested positive for Covid-19 yesterday while "experiencing no symptoms".

"With my recent positive test, I will begin the quarantine process and will continue working on behalf of the American People remotely," she tweeted.

"No reporters, producers, or members of the press are listed as close contacts by the White House Medical Unit," she added.

Vice-President Mike Pence and his wife Karen Pence again tested negative for Covid-19 yesterday, an administration official said.

Mr Pence was due to travel yesterday to Salt Lake City, Utah, where he is scheduled to hold the vice-presidential debate with Mr Biden's running mate, Senator Kamala Harris, tomorrow.