President Donald Trump has gone through a "very concerning" period and the next 48 hours "will be critical" in his coronavirus battle, the White House has said.
The comments by the White House chief of staff Mark Meadows on Saturday (local time) contradicted the rosy assessment of Trump's condition offered by his staff and doctors.
"We're still not on a clear path yet to a full recovery," he said.
The doctors treating the president had earlier updated the public at a press conference at Walter Reed National Military Medical Centre.
At the briefing, Navy Commander Dr Sean Conley repeatedly refused to say whether the president ever needed supplemental oxygen, and declined to discuss exactly when he fell ill.
Mr Conley also revealed that Mr Trump began exhibiting "clinical indications" of Covid on Thursday afternoon, earlier than previously known.
"Thursday no oxygen. None at this moment. And yesterday with the team, while we were all here, he was not on oxygen," Mr Conley said.
But according to a source, Mr Trump was administered oxygen at the White House on Friday before he was transported to the military hospital.
Mr Conley said Mr Trump's symptoms, including a cough and nasal congestion, "are now resolving and improving," and said the president had been fever-free for 24 hours. But Mr Trump also is taking aspirin, which lowers body temperature and could mask or mitigate that symptom.
Sean Dooley, another doctor, said Mr Trump remains in “exceptionally good spirits”.
Mr Trump is 74 years old and clinically obese, putting him at higher risk of serious complications from a virus that has infected more than seven million people nationwide and killed more than 200,000 people in the US.
Dr Conley said the president has “a lot of work to do” and is doing it.
Doctors said the president told them: “I feel like I could walk out of here today.”
On Saturday, 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!!!”
“With their help, I am feeling well!” he tweeted.
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