Dr. Kent Brantly was able to walk, with help, from an ambulance after he was flown on Saturday to Atlanta, where he is being treated by infectious disease specialists at Emory University Hospital.
"It's encouraging that he seems to be improving - that's really important - and we're hoping he'll continue to improve," said Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta.
Frieden told CBS's "Face the Nation" it was too soon to predict whether Brantly would survive, and a hospital spokesman said Emory did not expect to provide any updates on the doctor's condition on Sunday.
Brantly is a 33-year-old father of two young children who works for the North Carolina-based Christian organization, Samaritan's Purse. He was in Liberia responding to the worst Ebola outbreak on record when he contracted the disease.
Since February, more than 700 people in West Africa have died from Ebola, a hemorrhagic virus with a death rate of up to 90 percent of those infected. The fatality rate in the current epidemic is about 60 percent.
Frieden told ABC's "This Week" that the CDC was "surging" its response, and that it will send 50 staff to West Africa "to help stop the outbreak in the next 30 days."
Amber Brantly, Dr. Brantly's wife, said she was able to see her husband on Sunday and he was in good spirits, and that the family is confident he is receiving the very best care. "He thanked everyone for their prayers," she said in a statement.
A second U.S. aid worker who contracted Ebola alongside him, missionary Nancy Writebol, will be brought to the United States on a later flight. The medical aircraft is equipped to carry only one patient at a time.
Standard treatment for the disease is to provide supportive care. In Atlanta, doctors will try to maintain blood pressure and support breathing, with a respirator if needed, or provide dialysis if patients experience kidney failure, as some Ebola sufferers do.
SECOND MISSIONARY EXPECTED SOON
Writebol, a 59-year-old mother of two who worked to decontaminate those entering and leaving an Ebola isolation unit in Liberia, was due to depart for the United States overnight on Monday, Liberia's information minister said.
Writebol's husband, David, who had been living and working in Liberia with his wife, was expected to travel home separately in the next few days, their missionary organization, SIM USA, said in a statement.
Separately, the charity Medical Teams International said one of its doctors had placed himself in voluntary confinement after returning to the United States from Liberia on July 25.
The doctor, Alan Jamison, worked in the same isolation units as Brantly and Writebol, it said, adding that he has no symptoms and that there was no evidence he was exposed to the virus.
The facility at Emory chosen to treat the two infected Americans was set up with CDC and is one of four in the country with the ability to handle such cases.
The CDC has said it is not aware of any Ebola patient having been treated in the United States previously. Five people entered the country in the past decade with either Lassa Fever or Marburg, both hemorrhagic fevers similar to Ebola.
President Barack Obama has said some participants at an Africa summit in Washington this week would be screened for Ebola exposure. The CDC's Frieden said there was no reason to cancel the event.
"There are 50 million travelers from around the world that come to the U.S. each year ... We're not going to hermetically seal this country," he told Fox News Sunday.