An American healthcare worker who has been in Sierra Leone will be placed under close observation at a Nebraska hospital after high-risk exposure to the Ebola virus.

The unnamed patient will arrive on Sunday aboard a private air ambulance for observation and possible treatment.

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“This patient has been exposed to the virus but is not ill and is not contagious,” said Phil Smith, medical director of the Biocontainment Unit at Nebraska Medicine.

“However, we will be taking all appropriate precautions. This patient will be under observation in the same room used for treatment of the first three patients and will be carefully monitored to see if the Ebola disease develops.”

Two of the previous three Ebola patients at the facility were treated and released but the third – a surgeon who was infected while working in his native Sierra Leone – died in November.

Ebola has killed 7,890 people in the past year, out of 20,171 cases, according to the latest tally by the World Health Organisation.

Almost all the deaths and cases have been recorded in the three west African countries worst hit by the outbreak: Sierra Leone, Liberia and Guinea.