U.S. President Donald Trump, who is being treated for COVID-19 in a military hospital outside Washington, D.C., said in a video on Saturday that he was feeling ill when he arrived at the facility but now feels better.

Trump, looking tired and wearing a suit coat but no tie, said in a video released on his Twitter, "I'm starting to feel good." He added that "over the next period of a few days I guess that's the real test. We'll be seeing what happens over those next couple of days."

Trump was hospitalized Friday evening after experiencing a low-grade fever, chills, nasal congestion and cough, multiple sources with direct knowledge of the matter tell ABC News. 

His doctor said in an update on Saturday night that the medical team is "cautiously optimistic" on the president's health condition. "President Trump continues to do well, having made substantial progress since diagnosis," White House doctor Sean P. Conley said in a statement. "While not yet out of the woods, the team remains cautiously optimistic," the doctor added.

Meanwhile, U.S. Secretary of State Mike Pompeo will depart for Japan on Sunday but will not go to Mongolia and South Korea as originally planned, the State Department said on Saturday, after Trump was diagnosed and hospitalized due to COVID-19.

Precautionary measures

Trump will stay at the military hospital for the next few days after testing positive for COVID-19, the White House said in a statement on Friday, as his administration and election campaign scrambled to adjust to an extraordinary twist in his turbulent presidency. 

"Out of an abundance of caution, and at the recommendation of his physician and medical experts, the president will be working from the presidential offices at Walter Reed for the next few days," said White House Press Secretary Kayleigh McEnany, noting Trump has mild symptoms and remains in good spirits.

President Trump "appreciates the outpouring of support" for both him and the First Lady, the press secretary added.

The president was seen walking slowly from the White House to a waiting helicopter to be taken to Walter Reed National Military Medical Center in Bethesda, Maryland, roughly 17 hours after he announced that he had tested positive for the coronavirus. He wore a mask and business suit, and did not speak to reporters.

"I think I'm doing very well, but we're going to make sure that things work out," Trump said in a brief video posted to Twitter.

He said on Twitter late on Friday, after being hospitalized with COVID-19, that he thought he was doing well. "Going well, I think! Thank you to all. LOVE!!!," the president tweeted after having been moved to the military hospital for treatment.

Conley confirmed the news, saying Trump is "doing very well" and "is not requiring any supplemental oxygen."

The 74-year-old president has a mild fever though, according to a source familiar with the matter. Conley said earlier in a new memo that he is "fatigued but in good spirits" as of Friday afternoon.

Trump is considered to be at higher risk of developing severe COVID-19 because of his age and weight. He has remained in apparent good health during his time in office but is not known to exercise regularly or to follow a healthy diet.

He announced early Friday morning on his Twitter that he and his wife, Melania, had tested positive for COVID-19 shortly after confirming his close aide Hope Hicks had contracted the virus.

It was the latest setback for the Republican president, who is trailing behind Democratic rival Joe Biden in opinion polls ahead of the November 3 presidential election.

Read more:

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Treatment for Trump

Conley said that the president is taking Remdesivir therapy after consultation with specialists. "He has completed his first dose and is resting comfortably," he said.

He is also taking an antibody cocktail being tested by Regeneron Pharmaceuticals Inc, which has reportedly improved symptoms in non-hospitalized COVID-19 patients, with no serious side effects in trials. U.S. infectious disease chief Dr. Anthony Fauci is among those saying the technique has promise. 

"As a precautionary measure he received a single eight gram dose of Regeneron's polyclonal antibody cocktail. He completed the infusion without incident," Conley wrote. 

In addition, Conley said the president is taking several other medicines, including:

* Vitamin D and mineral zinc, both of which are important to the immune system and taken by some as immune system boosters.

* Heartburn drug famotidine, often sold under the brand name Pepcid. The drug has not been proven to work against COVID-19, but researchers are studying it as a potential treatment. 

* Melatonin, often taken as a sleep aid. 

* Aspirin, which Trump previously has said he takes daily. Aspirin can help avoid heart attacks and is a fever reducer. 

The doctor added the first lady "remains well with only a mild cough and headache" and that the remainder of the first family are well and tested negative for COVID-19 on Friday.

Dr. Jonathan Grein, director of hospital epidemiology at Cedars-Sinai Medical Center in Los Angeles, said the president is likely being closely monitored, and would be given extra oxygen or fluids if needed. 

'Be patriotic'

Stocks on Wall Street closed lower as news of Trump's diagnosis added to mounting uncertainties surrounding the election.

The president's illness upended his re-election campaign with just 31 days to go until Election Day. The campaign said it would postpone rallies and other events where he was scheduled to appear, or take them online.

Biden pulled ads attacking Trump off the air, but otherwise continued his campaign after testing negative for the virus.

At a union hall in Grand Rapids, Biden said he was praying for his rival's recovery. However, he also implicitly criticized Trump, who would rarely wear a mask in public and has held huge campaign rallies with little social distancing.

"Be patriotic," Biden said. "It's not about being a tough guy. It's about doing your part."

Trump's illness also raised questions about the health of others atop the U.S. government.

Read more: Biden wishes Trump, Melania 'swift recovery' from COVID-19

Replacement in line

Vice President Mike Pence, who would take over if Trump were to fall severely ill, has tested negative, a spokesperson said. The former Indiana governor, 61, is working from his own residence several miles from the White House.

Trump did not transfer power to Pence as other presidents have sometimes done while undergoing medical treatment, according to a White House official.

Pence took over Trump's planned calls with governors and retirees' organizations. His October 7 debate with Democratic vice presidential candidate Kamala Harris will go forward as planned, organizers said. Harris has also tested negative for the virus, according to the campaign.

House of Representatives Speaker Nancy Pelosi, third in line for the presidency, also had tested negative, a spokesperson said.

The Republican National Committee would chose a replacement nominee, if Trump were to become incapacitated, but it is already too late in most states to change the names on the ballot. Some 2.9 million people have already voted, according to figures compiled by University of Florida professor Michael McDonald.

The virus could also complicate Trump's push to install conservative judge Amy Coney Barrett on the Supreme Court.

At least two people who were at a White House event to announce her nomination, University of Notre Dame President John Jenkins and Republican Senator Mike Lee, said Friday they have tested positive.

Barrett herself tested positive for the virus earlier this year and has since recovered, according to a person familiar with the matter.

Trump is the latest world leader to be tested positive for the virus, which can cause a severe respiratory infection.

British Prime Minister Boris Johnson was rushed into intensive care after falling gravely ill in March. Brazilian President Jair Bolsonaro had called the virus a "little flu" and then was also infected in July. Both men recovered.

(With input from Xinhua, Reuters)

(Cover: Marine One helicopter with U.S. President Donald Trump onboard lifts off to depart the White House and fly to Walter Reed National Military Medical Center, Washington, D.C., U.S., October 2, 2020. /Reuters)