Confusion continued over the treatment of US President Donald Trump on Sunday when a specialist said that he could be released from the military hospital as early as Monday while his National Security Advisor said that he could stay there for "another period of time."

Trump had also had two short, temporary setbacks when he needed to be put on oxygen, his personal doctor Sean Conley disclosed at a Sunday morning briefing.

But he and other doctors gave an overall upbeat assessment of Trump's condition.

Sean Dooley, a military pulmonary critical care specialist, said that Trump "has remained without fever, since Friday morning, his vital signs are stable, from a pulmonary standpoint" and is walking around without "complaining of shortness of breath, or other typical respiratory symptoms".

A member of the medical team, Brian Garibaldi, a specialist from Johns Hopkins University, said: "If he continues to look and feel as well as he does today our hope is that we can plan for a discharge as early as tomorrow to the White House where he can continue his treatment course."

But earlier National Security Advisor Robert O'Brien told a CBS TV programme that although Trump feels "very well" and wants to return to the White House, "I think he's going to stay at Walter Reed for at least another period of time."

O'Brien, who had come down with Covid-19 and has recovered, said: "Days seven and eight are the critical days." With elections 30 days away, Trump's campaign is hobbled by his illness. Vice President Mike Pence is set to debate Democratic Party's vice-presidential nominee Kamala Harris on Tuesday.

Her campaign has asked for a 12-foot separation between them on the stage given Pence's likely exposure to Trump. Three Republican senators, Republican National Committee Chair Ronna McDaniel and Trump's advisers, Kellyanne Conway and former New Jersey Governor Chris Christie, are among those who have come down with the virus.

Conley, who had been evasive on Trump's oxygen use, said that "Over the course of his illness, the President has experienced two episodes of transient drops in his oxygen saturation".

Late Friday morning at the White House, Conley said, that he found Trump had a high fever and his oxygen saturation was below 94 per cent and gave him supplemental oxygen.

"Fortunately, that was really a very transient limited episode, a couple hours later, he was back up," he said. He said that Trump's oxygen levels, a key indicator of a Covid-19 patient's status, did not dip into the 80s at any time.

Conley said that Trump was given a powerful steroid, dexamethasone, on Saturday. That, like remdisivir with which Trump is being treated, is an experimental medication for the treatment of Covid-19.

Asked about Trump's Chief of Staff Mark Meadows saying on Saturday that Trump's "vitals over the last 24 hours were very concerning," Conley said that the remark was "misconstrued" and that he was only referring to the specific temporary incident on Friday morning, which he said prompted the decision to fly him by helicopter him to the hospital.

In a video made at the hospital and released on Twitter on Saturday, Trump 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."