Donald Trump went through a “very concerning” period on Friday and faces a “critical” next two days in his fight against Covid-19 at a military hospital, his chief of staff said.

Mark Meadows’ comments contrasted with a rosier assessment moments earlier by doctors treating the US president, who were at pains not to reveal that Mr Trump had received supplemental oxygen at the White House before being admitted to hospital.

Mr Trump offered his own assessment in a video from the Walter Reed National Military Medical Centre on Saturday evening, saying he was beginning to feel better and hoped to “be back soon”.

And he was back on social media early Sunday morning, sharing a video of flag-waving supporters, most not wearing masks, gathered outside the medical centre.

“Thank you so much!” Mr Trump tweeted.

The day before, Mr Meadows told reporters outside the hospital in Bethesda, Maryland: “We’re still not on a clear path yet to a full recovery.”

In an update on Mr Trump on Saturday night, the president’s chief doctor expressed cautious optimism but added that he is “not yet out of the woods”.

The changing, and at times contradictory, accounts created a credibility crisis for the White House at a crucial moment, with the president’s health and the nation’s leadership on the line.

With Mr Trump expected to remain in hospital for several more days and the presidential election looming, his condition is being anxiously watched by Americans.

Moreover, his health represents a national security issue of paramount importance not only to the functions of the US government but to countries around the world, friendly and otherwise.

Saturday’s briefing by Navy Commander Dr Sean Conley and other doctors raised more questions than it answered.

Dr Conley repeatedly refused to say whether the president had needed supplemental oxygen, despite repeated questioning, and declined to share key details including how high a fever Mr Trump had been running before it came back down to a normal range.

Dr Conley also revealed that Mr Trump had begun exhibiting “clinical indications” of Covid-19 on Thursday afternoon, earlier than previously known.

The medic spent much of the briefing dodging reporters’ questions, as he was pressed for details.

“Thursday no oxygen. None at this moment. And yesterday with the team, while we were all here, he was not on oxygen,” he said.

But according to a person familiar with Mr Trump’s condition, the president was given oxygen at the White House on Friday morning, well before he was transported to the military hospital by helicopter that evening.

Dr Conley said Mr Trump’s symptoms, including a mild cough, nasal congestion and fatigue, “are now resolving and improving”, and that the president had been fever-free for 24 hours.

But Mr Trump is also taking aspirin, which lowers body temperature and could mask or mitigate that symptom.

“He’s in exceptionally good spirits,” said another doctor, Sean Dooley, who said Mr Trump’s heart, kidney, and liver functions are normal and that he is not having trouble breathing or walking around.

In an evening health update, Dr Conley said Mr Trump had been up and moving around his medical suite without difficulty and conducting business.

“While not yet out of the woods, the team remains cautiously optimistic,” he said.

In the hospital video, Mr Trump defended his decision to continue campaigning and holding large events in the midst of a pandemic.

“I had no choice,” said the president, who had refused to abide by basic public health recommendations, including wearing a face covering. “I had to be out front … I can’t be locked up in a room upstairs and totally safe … As a leader, you have to confront problems.”

Mr Trump also thanked his medical team and hailed the state-of-the-art treatments he was receiving, comparing them to “miracles coming down from God”.

His medical care is far superior to that of the average American, with round-the-clock attention and experimental treatments.

The president was angry at Mr Meadows’ public assessment of his health and, in an effort to prove his vitality, ordered the video and authorised longtime confidant Rudy Giuliani to release a statement on his behalf that he was feeling well, according to a Republican close to the White House.

Mr Trump is 74 and clinically obese, putting him at higher risk of serious complications from the virus that has infected more than seven million people nationwide and killed more than 200,000 people in the US.

First Lady Melania Trump remained at the White House to recover from her own bout with the virus.

She is “really handling it very nicely”, Mr Trump said in the video, noting with a touch of humour that she is “just a little tiny bit younger” — 24 years younger, in fact.

Mr Meadows himself had insisted on Friday morning that Mr Trump had only “mild symptoms” as the White House tried to project an image of normality. It was unclear whether the president had already received oxygen when the chief of staff spoke.

“President Trump remains in good spirits, has mild symptoms and has been working throughout the day,” press secretary Kayleigh McEnany said late on Friday. She said Mr Trump had only been sent to Walter Reed as a precaution.

Mr Trump’s administration has been less than transparent with the public throughout the pandemic, both about the president’s health and the spread of the virus inside the White House.

The first word that a close aide to Mr Trump had been infected came from the media, not the White House. And aides have repeatedly declined to share basic health information, including full details of the president’s symptoms, what tests he has undertaken and the results.

In a memo released late on Friday, Dr Conley did report that Mr Trump had been treated in hospital with remdesivir, an antiviral medication, after sharing that he had taken another experimental drug at the White House.

The medic declined to say when the president had last been tested before he was confirmed to have Covid-19 late on Thursday. He initially suggested that Mr Trump was 72 hours into the diagnosis – which would mean that the infection was confirmed on Wednesday.

Dr Conley later clarified that Mr Trump was given an accurate test for the virus on Thursday afternoon, after White House aide Hope Hicks was confirmed to be positive and the president exhibited “clinical indications” of the virus.

The White House has said Mr Trump is expected to stay at the hospital for “a few days” and will continue to work from its presidential suite, which is equipped to allow him to keep up his official duties.

In addition to accessibility to tests and equipment, the decision to move him to the hospital on Friday was made, at least in part, with the understanding that hurrying there later could send a worrying signal if he took a turn for the worse.

On Saturday, Dr Conley said Mr Trump’s blood oxygen level was 96%, which is in the normal range.

The two experimental drugs he has received, given intravenously, have shown some promise against Covid-19. On Friday, he was given a single dose of a drug Regeneron Pharmaceuticals is testing to supply antibodies to help his immune system fight the virus.

On Friday night, he began a five-day course of remdesivir, a Gilead Sciences drug currently used for moderately and severely ill patients.

The drugs work in different ways – the antibodies help the immune system rid the body of virus and remdesivir curbs the virus’ ability to multiply.

“We’re maximising all aspects of his care,” attacking the virus in multiple ways, Dr Conley said. “I didn’t want to hold anything back if there was any possibility it would add value to his care.”

He noted that, in many cases, Covid-19 can become more dangerous as the body responds.

“The first week of Covid, and in particular day seven to 10, are the most critical in determining the likely course of this illness,” he said.

At the same time, the White House has been working to trace a flurry of new infections of close Trump aides and allies.

Attention is focused in particular on the White House event introducing Mr Trump’s Supreme Court nominee on September 26.

That day, the president gathered more than 150 people in the Rose Garden, where they mingled, hugged and shook hands — overwhelmingly without masks.

There were also several indoor receptions, where Mr Trump’s Supreme Court choice, Judge Amy Coney Barrett, her family, senators and others spent time in the close quarters of the White House, photographs show.

Among those who attended and have now tested positive are former New Jersey Governor Chris Christie, White House counsellor Kellyanne Conway, the president of the University of Notre Dame, and at least two Republican politicians – Utah Senator Mike Lee and North Carolina Senator Thom Tillis.

The president’s campaign manager, Bill Stepien, and head of the Republican National Committee Ronna McDaniel have also tested positive, though they were not at the event. Another prominent Republican who has tested positive is Wisconsin Senator Ron Johnson.

One of the president’s personal assistants, Nick Luna, tested positive after having travelled with Mr Trump several times recently, a White House official said on Saturday night.

Despite its failure to protect the president and senior staff from infection, the White House has given no indication that it intends to make any major protocol changes, such as making it mandatory for everyone to wear a mask.

Chief of staff Mr Meadows accompanied the president to hospital on Marine One, the kind of small, enclosed space where experts say the virus easily spreads. Those on board did wear masks.

While Vice President Mike Pence is currently off the campaign trail preparing for the coming week’s vice presidential debate, he and his staff are operating under a “business as usual” approach.

He is still planning to travel to Arizona on Thursday, Indiana on Friday and Florida on Saturday for events instead of isolating himself after potential exposure and to protect himself from contracting the virus anywhere else.