(Reuters) - At least 10 Americans possibly exposed to the deadly Ebola virus were being flown to the United States from Sierra Leone for observation, the U.S. Centers for Disease Control and Prevention said on Saturday.

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

All of the individuals who are being flown back to the United States are free of symptoms, the CDC said.

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.

It is not clear how the person became infected with Ebola, 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.

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 10 individuals will follow the center's recommended monitoring and movement guidelines during the 21-day incubation period.

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

On Friday, CDC sent a team to Sierra Leone to investigate how the healthcare worker became exposed, and determine who might have been in contact with the infected person.

CDC spokesman Benjamin Haynes did not know where all of the patients would be sent, but he said the CDC is working out a plan with the U.S. State Department to determine who is coming back and where they will be sent.

The CDC said one patient was being sent to Emory University Hospital's special isolation unit, where several Ebola patients have already been treated.

Four others are being sent to Nebraska Medical Center to be near their special isolation unit in case they develop Ebola symptoms.

(Reporting by Eric Beech in Washington and Julie Steenhuysen in Chicago; Editing by Marguerita Choy and Paul Simao)