WASHINGTON (Reuters) - A United States healthcare worker who tested positive for the Ebola virus while working in Sierra Leone arrived in the United States for treatment on Friday, US health officials said.

The patient, who was transported in isolation by chartered aircraft, was admitted to the US National Institutes of Health's (NIH's) high-security containment facility in Maryland, NIH said in a statement. "The patient's condition is still being evaluated," it said.

The unidentified American arriving on Friday is being treated at the NIH Clinical Centre's Special Clinical Studies Unit at the agency's campus in Bethesda, Maryland, just outside Washington.

NIH last year treated Texas nurse Nina Pham, who became infected with the Ebola virus while working with a patient at Texas Health Presbyterian Hospital in Dallas. She subsequently recovered. It also cared for two people who were exposed to Ebola while working in West Africa, but were later found not to be infected.

Several infected Americans working in West Africa, the centre of the Ebola outbreak, have returned to the United States for treatment since last summer. The virus has killed nearly 10,000 people in Sierra Leone, Liberia and Guinea.