WASHINGTON (AFP) - Several American healthcare workers who may have come into contact with a US volunteer who tested positive for Ebola in Sierra Leone are being monitored for signs of illness, officials said Friday.

The patient, whose identity has not been revealed, was in serious condition after arriving by private charter plane at the National Institutes of Health Clinical Centre near the US capital early Friday.

No others have tested positive so far, but at least one other American is being flown from Sierra Leone to Atlanta, Georgia as a precautionary measure, the Centres for Disease Control and Prevention (CDC) said.

Out of an abundance of caution, authorities are tracking down people who came into contact with the patient in Sierra Leone, "including several other American citizens, who may have had potential exposure to this index patient," the CDC said in a statement.

"At this time, none of these individuals have tested positive for Ebola. These individuals are volunteers in the Ebola response and are currently being monitored in Sierra Leone."

The CDC and the US State Department "are developing contingency plans for returning those Americans with potential exposure to the US by non-commercial air transport," it added.

Upon return, they will "voluntarily self-isolate and be under direct active monitoring for the 21-day incubation period".

Already, one of the Americans with possible exposure is "currently being transported via charter to the Atlanta area to be close to Emory University Hospital," which has treated US patients with Ebola in the past.

"The individual has not shown symptoms of Ebola and has not been diagnosed with Ebola. Upon arrival in Atlanta, the individual will voluntarily self-isolate and be under direct active monitoring for the 21-day incubation period," the CDC said.

Ebola is spread through close contact with the bodily fluids of an infected person. The virus can cause lethal bleeding, muscle aches, severe vomiting and diarrhoea.

The illness usually takes hold two to 10 days after exposure, but the full inoculation period is considered to last 21 days.

The index patient - whose identity has not been revealed - is at the NIH Clinical Centre in Bethesda, Maryland, which has a high-level isolation unit and a staff of infectious disease specialists.

The same facility treated US nurse Nina Pham, who was infected while caring for a Liberian man at a Texas hospital.

The man, Thomas Eric Duncan, died. Pham and another nurse who was also infected have since recovered.

On Thursday, the World Health Organisation announced that the death toll from the world's largest Ebola outbreak had topped 10,000.

Most of the deaths in the outbreak, which began in late 2013, have been in the West African nations of Sierra Leone, Guinea and Liberia.