U.S. President Donald Trump, who is currently in hospital for COVID-19 treatment, sought to reassure the nation on Saturday, as his medical team remained "cautiously optimistic" about his condition.

In a four-minute video clip posted on Twitter this evening, Trump said he's feeling "much better" and that he thinks he will "be back soon."

"I came here. Wasn't feeling so well. I feel much better now, we're working hard to get me all the way back," said the president, seen sitting behind a desk, wearing a blue jacket and a white shirt with the collar button undone, adding that the coming days would be critical for his health.

"Over the next period of a few days, I guess that's the real test," he noted. "So we'll be seeing what happens over those next couple of days."

Trump was first diagnosed with COVID-19 on Thursday and moved to Walter Reed National Military Medical Center in Bethesda, Maryland on Friday, where the president was using Remdesivir, a COVID-19 drug made by the U.S. biopharmaceutical company Gilead Sciences, as a treatment. He also received a dose of an experimental antibody cocktail being developed by U.S. drug maker Regeneron.

White House physician Sean Conley said Saturday that Trump has "made substantial progress" since the diagnosis and completed his second dose of Remdesivir that evening without complication.

"He remains fever-free and off supplemental oxygen with a saturation level between 96 and 98% all day," the doctor wrote. "While not yet out of the woods, the team remains cautiously optimistic."

Conley also revealed that Trump had spent most of the afternoon conducting business and has been up and moving about the medical suite at Walter Reed "without difficulty."

The assessment came hours after a source familiar with Trump's health reveals that they are "still not on a clear path to a full recovery."

"The president's vitals over the last 24 hours were very concerning," the source said in comments passed along to the media by a member of the White House press pool. "The next 48 hours will be critical in terms of his care."

Trump, aged 74 and clinically obese, is at a higher risk of serious complications from COVID-19 that has infected more than 7.3 million people and killed nearly 210,000 in the United States.

In an interview with Fox News, White House chief of staff Mark Meadows said both he and Conley were "real concerned" about Trump on Friday morning when the president "had a fever, and his blood oxygen level dropped rapidly."

Meadows also claimed doctors made the recommendation that Trump, who reportedly received supplemental oxygen that day, should travel to Walter Reed "out of an abundance of caution," pointing out that "there was never a consideration and never even a risk with the transition of power."

Trump's wife, first lady Melania Trump, also tested positive for COVID-19. She is in quarantine at the White House and is said to have been "doing very well," according to the president.

It is not yet clear when and how the Trumps infected with the virus. The president announced his infection early Friday morning shortly after confirming White House counselor Hope Hicks, who had traveled with him multiple times in the past week, had caught the disease.

Before the diagnosis, Trump had a packed schedule, including hosting a ceremony to introduce a Supreme Court nominee at the White House Rose Garden, traveling to Cleveland, Ohio for the first 2020 presidential debate, addressing a rally in Duluth, Minnesota, and attending a fundraiser at his New Jersey golf club.

The president reportedly got his first positive coronavirus test result Thursday evening after returning from the fundraiser. That result was via a rapid test. He then took a more thorough PCR test, used to directly detect the presence of an antigen, which also came back positive.

A growing number of individuals in Trump's orbit, including three Republican senators, campaign manager Bill Stepien, and former New Jersey Governor Chris Christie who helped the president prepare for the debate on Tuesday, have also contracted COVID-19.

Christie tweeted Saturday evening that he has checked himself into a hospital in Morristown, New Jersey.

"While I am feeling good and only have mild symptoms, due to my history of asthma we decided this is an important precautionary measure," he wrote.