WASHINGTON (AFP) - A 33-year-old American photojournalist who became infected with Ebola while working in Liberia has shown a modest improvement in his health in recent days, hospital officials said Friday.

Ashoka Mukpo arrived at Nebraska Medical Centre on Oct 6, and was able to walk off the plane that evacuated him from west Africa.

He remained in stable condition in the days that followed, said a hospital statement.

On Friday, the hospital said he had "shown a very modest improvement from his condition over the past few days."

"Mr Mukpo's condition is slightly improved," said Phil Smith, medical director of the Biocontainment Unit at the Nebraska Medical Centre.

"He's been taking in some fluids and drinking Gatorade. But everyone needs to be reminded that this is still a very serious illness we're dealing with and no one has a lot of experience treating it."

Mukpo received an experimental antiviral drug, brincidofovir, and a blood transfusion from Ebola survivor Kent Brantly, a missionary doctor who was sickened with the virus over the summer.

"I'm cautiously optimistic," said Mitchell Levy, Mukpo's father.

"We definitely aren't out of the woods, but it's nice to see even a small amount of improvement."

Mukpo is the fifth American who was infected with Ebola in West Africa and whose return to the United States was aided by the State Department.

He was working as a freelance cameraman for NBC News in Liberia before he fell ill with the virus that has killed more than 4,000 people in West Africa since the beginning of the year.

The first American to be infected with Ebola, Patrick Sawyer, a dual US-Liberian citizen, died of his infection in July after travelling by plane from Liberia to Nigeria.

On Wednesday, Thomas Eric Duncan, a Liberian who was the first person diagnosed with Ebola in the United States, died of the illness in a Texas hospital.