BETHESDA — President Donald Trump went through a "very concerning" period Friday and faces a "critical" next two days in his fight against COVID-19 at a military hospital, his chief of staff said Saturday — in contrast to a rosier assessment moments earlier by Trump doctors, who took pains not to reveal the president had received supplemental oxygen at the White House before his hospital admission.

Trump offered his own assessment Saturday evening in a video from Walter Reed National Military Medical Center, saying he was beginning to feel better and hoped to "be back soon."

Hours earlier, chief of staff Mark Meadows told reporters outside the hospital, "We're still not on a clear path yet to a full recovery."

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 Trump expected to remain hospitalized several more days and the presidential election looming, his condition is being anxiously watched by Americans.

Moreover, the president's 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. Conley repeatedly refused to say whether the president ever needed supplemental oxygen, despite repeated questioning, and declined to share key details including how high a fever Trump had been running before it came back down to a normal range. Conley also revealed that Trump had begun exhibiting "clinical indications" of COVID-19 on Thursday afternoon, earlier than previously known.

Conley 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," Conley said.

But according to a person familiar with Trump's condition, Trump was administered oxygen at the White House on Friday morning, well before he was transported to the military hospital by helicopter that evening. The person was not authorized to speak publicly and spoke to The Associated Press only on condition of anonymity.

Conley said that Trump's symptoms, including a mild cough, nasal congestion and fatigue "are now resolving and improving," and said the president had been fever-free for 24 hours. But Trump also is 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 Trump's heart, kidney, and liver functions were normal and that he was not having trouble breathing or walking around.

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

"I had no choice," said Trump, who refused to abide by basic public health recommendations, including mask-wearing. "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."

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." Trump's medical care is far superior to the average American's, with around-the-clock attention and experimental treatments.

Trump is 74 years old and clinically obese, putting him at higher risk of serious complications from a virus that has infected more than 7 million people nationwide and killed more than 200,000 people in the US.

Meadows himself had insisted Friday morning that Trump had only "mild symptoms" as the White House tried to project an image of normalcy. It was unclear whether Trump already had received oxygen when Meadows spoke.

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

Trump's administration has been less than transparent with the public throughout the pandemic, both about the president's health and the virus' spread inside the White House. The first word that a close aide to Trump had been infected came from the media, not the White House. And aides have repeatedly declined to share basic health information, including a full accounting of the president's symptoms, what tests he's undertaken and the results.

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

Conley declined to say when Trump had last been tested before he was confirmed to have COVID-19 late Thursday. He initially suggested that Trump was 72 hours into the diagnosis — which would mean that he was confirmed infected Wednesday. Conley later clarified that Trump was administered an accurate test for the virus on Thursday afternoon, after White House aide Hope Hicks was confirmed to be positive and Trump exhibited "clinical indications" of the virus.

The White House has said Trump was expected to stay at the hospital for "a few days" and would 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 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.