An American healthcare worker who contracted Ebola while volunteering in a Sierra Leone treatment unit arrived safely at the National Institutes of Health's hospital in Maryland, officials announced early Friday.

The patient's condition was still being evaluated after being flown in isolation from Sierra Leone on a chartered plane and admitted at 4:44 a.m. local time, NIH officials said in a statement. The patient's name, age and gender were not released.

The patient is the 11th person with Ebola to be treated in the U.S. and the second admitted to the NIH Clinical Center. It has one of the few containment facilities nationwide set up to treat Ebola patients.

The centre's Special Clinical Studies Unit is staffed by specialists in infectious disease and critical care and is designed to prevent the spread of highly contagious viruses, including Ebola. 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. The current outbreak is the largest ever for the disease. While deaths have slowed dramatically in recent months, the virus appears stubbornly entrenched in parts of Guinea and Sierra Leone.