(Reuters) - The U.S. National Institutes of Health plans to admit to one of its special observation wards an American physician exposed to the Ebola virus while volunteering in Sierra Leone, it said on Saturday.

The patient, who has not been identified, was expected to be admitted on Sunday to the NIH Clinical Center in Bethesda, Maryland, for observation and to enroll in a clinical study, the institute said.

"Out of an abundance of caution, the patient will be 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.

The death toll from an outbreak of Ebola in West Africa has risen to at least 3,091 out of 6,574 probable, suspected and confirmed cases, the World Health Organization said on Friday.

The outbreak that began in a remote corner of Guinea has taken hold of much of neighboring Liberia and Sierra Leone, prompting warnings that tens of thousands of people may die from the worst outbreak of the disease on record.

The third U.S. patient to be treated in the United States for Ebola is now free of the virus, doctors at the Nebraska Medical Center, where the patient was being treated, said in a news conference earlier this week.

(Reporting by Jon Herskovitz in Austin, Texas; Editing by Peter Cooney)