At least 10 U.S. citizens possibly exposed to the deadly Ebola virus were being flown to the United States from Africa for observation, the U.S. Centers for Disease Control and Prevention said on Saturday.

The individuals will be transported by non-commercial air transport and will be housed near the University of Nebraska Medical Center, the National Institutes of Health in Maryland, or Emory University Hospital in Atlanta, the CDC said.

It said none of the individuals have been identified as having Ebola.

A U.S. healthcare worker who tested positive for Ebola while in Sierra Leone arrived at the NIH on Friday and was in serious condition, the NIH said.

CDC spokesman Thomas Skinner said 10 people who may have been exposed to the unidentified Ebola patient or who had a similar exposure to the virus as the patient were being flown to the United States. But he said the investigation was continuing and there may be more Americans evacuated from Africa.

A CDC statement said the individuals will follow the centre's recommended monitoring and movement guidelines during a 21-day incubation period.

If an individual shows symptoms, they will be transported to an Ebola treatment centre for evaluation and care, the CDC said.

While the virus has killed about 10,000 people in Sierra Leone, Liberia and Guinea, only a handful of cases have been seen in the United States, Spain and Britain.