WASHINGTON, Sept. 28 -- A fifth American patient infected with the Ebola virus in West Africa was brought home Sunday afternoon and admitted to the U.S. National Institutes of Health (NIH), the medical research agency said.

The physician, who has been working in an Ebola treatment unit in Sierra Leone, was "transferred from an overseas location" and arrived at the NIH clinical center at about 4 p.m. EDT (2000 GMT), the NIH said in a statement.

"Out of an abundance of caution, the patient has been admitted to the NIH Clinical Center's special clinical studies unit that is specifically designed to provide high-level isolation capabilities and is staffed by infectious diseases and critical care specialists," it said.

No additional details about the patient were available at this time, it added.

The institute said on Saturday that the patient, who has not been identified, was expected to enroll in a clinical study after his admission to the NIH Clinical Center in Bethesda, Maryland, for observation.

Four American aid workers have already been airlifted back to the United States from West Africa. All but one have already recovered and been released from hospital.

The current Ebola outbreak in West Africa has killed more than 3,000 people and more than 6,500 people are believed to have been infected in the region, the World Health Organization said on Friday.