Video report by ITV News Washington Correspondent Robert Moore
Confusion is building over the condition of President Donald Trump after he and doctors said he is "doing well" following his coronavirus diagnosis, but his chief of staff told reporters the next 48 hours "will be critical" if he is to fully recover.
The comments came after it was revealed that Mr Trump was administered supplemental oxygen on Friday morning at the White House before he was transported to a military hospital, although staff insisted he had only mild symptoms.
The president tweeted to say he is “feeling well” thanks to the medics treating him at Walter Reed National Military Medical Centre, but White House chief of staff Mark Meadows struck a less optimistic tone.
He revealed Mr Trump went through a “very concerning” period on Friday following his coronavirus diagnosis, adding: "We’re still not on a clear path yet to a full recovery."
In a press conference aimed at clarifying the president's condition, his physician Dr Sean Conley said he is "doing very well".
The president's medical team said they are “extremely happy” with his progress, with many of his coronavirus symptoms - which include a cough and nasal congestion - “now resolving and improving.”
After the update, Mr Trump tweeted: “Doctors, Nurses and ALL at the GREAT Walter Reed Medical Center, and others from likewise incredible institutions who have joined them, are AMAZING!!!
"Tremendous progress has been made over the last 6 months in fighting this PLAGUE. With their help, I am feeling well!”
But after the press conference a White House source told reporters the president's vital signs over the past 24 hours had caused severe concern, adding that the next 24 hours would be "critical" for Mr Trump.
At the press conference it was revealed Mr Trump told doctors on Saturday morning he felt "like I could walk out of here today".
Amid repeated questioning, Navy Commander Dr Conley was reluctant confirm whether the president had needed oxygen treatment but they said he is not currently receiving help breathing.
It was later revealed, according to an Associated Press source, that Mr Trump was administered supplemental oxygen at the White House on Friday before going to hospital.
Mr Trump was taken by helicopter to the hospital “out of an abundance of caution” on Friday, and is expected to remain there for “a few days”.
Dr Conley said the president, who is being treated with Remdesivir, may be allowed to go back to the White House soon if he is able to be taken off the drug.
“He’s up and active and able to handle it," the doctor said.
First lady Melania Trump has also tested positive and has a “mild cough and headache”, according to the doctor, but the remainder of the first family, including son Barron, who lives at the White House, have tested negative.
The US President gave a thumbs up to reporters as he walked out of the White House and was flown by helicopter to the Walter Reed National Military Medical Centre on Friday.
The White House said Mr Trump would continue to work from the hospital's presidential suite.
After his arrival at the hospital, Trump tweeted: "Going well, I think! Thank you to all. LOVE!!!"
Prior to his departure from the White House, he posted a pre-recorded video thanking "everyone for the tremendous support".
He said: "I’m going to Walter Reed Hospital. I think I’m doing very well, but we’re going to make sure that things work out.
"The first lady is doing very well, so thank you very much I appreciate it, I will never forget it."
Responding to the news the Democratic presidential nominee Joe Biden tweeted: "This cannot be a partisan moment. It must be an American moment. We have to come together as a nation."
Boris Johnson wished the president a full recovery, telling reporters he had spoken to members of the Trump family.
He said: "I think obviously everybody's wishing him and Melania the very best and hoping that they recover speedily and I've no doubt that he will, he'll make a very strong recovery."
When asked if he had any advice for Mr Trump, Mr Johnson said: "I think that he will be doing exactly what the doctors tell him to do and I'm sure he will be having the best care he can possibly get."
He added: "I'm sure he'll come through it very well."
The president’s physician said in a memo that Trump received a dose of an experimental antibody combination by Regeneron that is in clinical trials.
Navy Commander Dr Conley said Trump “remains fatigued but in good spirits” and that a team of experts was evaluating both the president and first lady in regard to next steps.
Late on Friday Dr Conley issued an update that said Trump is “doing very well” and is “not requiring any supplemental oxygen,” but he said that, “in consultation with specialists we have elected to initiate Remdesivir therapy,” an antiviral medication.
“He has completed his first dose and is resting comfortably,” the doctor wrote.
ITV News Washington Correspondent Robert Moore discusses what will happen if the president is too ill to work
The positive tests for the Trumps come as stark reminders of the continued coronavirus pandemic, even as Mr Trump has tried to suggest it no longer poses a danger.
His admission to hospital came less than 24 hours after it was confirmed Mr Trump had tested positive for the virus.
The USA currently has the highest number of confirmed cases worldwide, and the highest death toll of any country across the globe.
All campaign events featuring president and his family have been postponed after he and the First Lady tested positive.
Mr Biden, who has tested negative for Covid-19, said earlier the president's diagnosis served as a "bracing reminder to all of us that we have to take this virus seriously", adding he and wife Jill are praying the Trumps make a quick and full recovery.
He encouraged everyone to follow the science, practice social distancing and wear a mask, adding "it's not about being a tough guy."
"We can save 100,000 American lives in the next 100 days alone if everyone wore a mask in public," Mr Biden said, citing researchers from the University of Washington.
Mr Biden confirmed earlier both he and wife Jill have tested negative for Covid-19 - they were tested in light of the president's confirmed coronavirus diagnosis.
Vice-president Mike Pence - who has tested negative for Covid - will continue on the campaign trail in Mr Trump's absence.
The Trumps' positive tests follow the announcement on Thursday that the president's senior aide, Hope Hicks, had tested positive for Covid.
The World Health Organization's director-general wished the couple "a full and swift" recovery.
Speaking from Geneva, Tedros Adhanom Ghebreyesus said his prayers are with the president and the first lady.