Concerns were rising over US President Donald Trump's health condition after his doctor said that he was "not yet out of the woods" following his Covid-19 diagnosis, while the 74-year-old leader also acknowledged that the next few days will be a "real test".

"While not yet out of the woods, the (medical) team remains cautiously optimistic," his doctor Sean Conley said on Saturday night in a memo released through Trump's Spokesperson Kayleigh McEnany.

Also in a video message, the President said: "Over the next period of a few days, I guess that's the real test. So we'll be seeing what happens over those next couple of days."

The video was made at the Walter Reed National Medical Centre where he is undergoing treatment for Covid-19.

After a briefing by doctors at the hospital on Saturday morning, Trump's Chief of Staff Mark Meadows told reporters: "We're still not on a clear path to a full recovery. The President's vitals over the last 24 hours were very concerning and the next 48 hours will be critical in terms of his care."

Trump has three high-risk factors for the coronavirus as a 74-year-old male who is overweight at 110 kg.

Thirty days are left for the November 3 election and Trump, who had controversially crisscrossed the country holding closely-packed rallies without wearing a mask, will have to come up with new strategy to campaign.

His Democratic rival Joe Biden, who had campaigned mostly electronically from his home for till last month, is able to campaign in person as he did on Friday in Michigan.

Trump was admitted to the Walter Reed National Military Medical Centre in Washington on Friday evening after tests the previous day had shown him positive for Covid-19.

In his press briefing, Conley said that Trump spent most of Saturday afternoon "conducting business" and the medical team at the Walter Reed military hospital "will be closely monitoring his clinical status, while supporting his conduct of presidential duties" on Sunday.

He added that Trump "has been up and moving about the medical suite without difficulty".

The White House released pictures on Saturday night showing him at work at the hospital that has a presidential suite and facilities for him to work.

Trump's four-minute video released on Twitter showed him seated at table with his hands resting on it and dressed casually with a blue jacket.

His voice sounded firm, but slightly scratchy.

Explaining his move to the hospital, Trump said that he chose not to be "locked up in a room upstairs and totally safe" because "we have to confront problems. As a leader, you have to confront problems".

Conley said that Trump completed a second dose of the experimental medicine Remdesivir on Saturday without complications and will continue with it on Sunday.

Earlier on Saturday, there was confusion in the timeline of Trump's infection.

Conley told reporters that Trump was found to be positive 72 hours earlier, which would have put it on Wednesday, whereas the President had said he was tested on Thursday and announced the result early Friday.

Later, Conley issued a clarification that he meant to say that Saturday was "day three" following the diagnosis on Thursday and not "72 hours".

Two important issues facing Trump have come under a cloud of uncertainty after three Senators from his party came down with the virus.

His tight timeline to have his Supreme Court nominee Amy Coney Barrett confirmed by the Senate before the election received a setback because of the illness of the senators, two of whom are members of the Senate Judiciary Committee which was to have started the process on October 12.

The Democratic Party-controlled House of Representatives has passed a $2.2 trillion relief package to help individuals, businesses, states and local governments deal with the impact of the pandemic.

The Republicans, who control the Senate, oppose it, but the two sides will have to make a compromise before the election and get it through the Senate.

Its next meeting has been postponed to October 19 because of the infections..