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Sierra Leone has recorded 11 696 Ebola cases and 3663 deathsJohn Moore/Getty Images

An American healthcare worker who contracted Ebola while volunteering in a Sierra Leone treatment unit has arrived at the National Institutes of Health's hospital in Maryland.

The patient, who was flown in isolation from Sierra Leone on a chartered plane, is in serious condition.

He was admitted at 4:44 am, NIH officials said in a statement on Friday.

The patient's name, age and gender were not released.

The patient is the 11th person with Ebola to be treated in the US, and the second admitted to the NIH Clinical Center, which has one of the few specialized isolation units nationwide set up to treat Ebola patients.

Previously, an American nurse was treated there after she contracted Ebola while caring for a patient in a Dallas hospital. The nurse, Nina Pham, survived and is Ebola-free.

The World Health Organization estimated Thursday that the virus has killed more than 10,000 people, mostly in the West African nations of Guinea, Liberia and Sierra Leone.