Ebola, Nebraska Medical Center
The Nebraska Medical Center in Omaha.Reuters

The US has started monitoring a health worker possibly exposed to the Ebola virus while working in Sierra Leone.

The patient was transported to the country on Sunday in a private air ambulance and has been admitted to the Nebraska Medical Centre in Omaha for observation and testing.

"A patient who was possibly exposed to Ebola is now undergoing testing here," the hospital tweeted on 4 January.

This is the fourth Ebola case the hospital has received. The Nebraska Medical Centre is one of the four specialised bio-containment facilities in the US designed to treat Ebola patients. It has already treated and cured Dr Rick Sacra, a medical missionary from Massachusetts and Ashoka Mukpo, a freelance cameraman of NBC News. Both contracted the infection while working in Liberia.

However, the hospital couldn't save Dr Martin Salia, a surgeon who was critically ill when he was transported from Sierra Leone in November. Reports showed that the 44-year-old patient, who was a permanent resident of the US, received proper treatment nearly two weeks after he started exhibiting the symptoms.

In a statement released by the hospital, authorities revealed that the new patient has not started exhibiting any symptoms.

The patient is under the care and supervision of an expert team and will be kept under observation for a period of 21 days.

"This patient has been exposed to the virus but is not ill and is not contagious," Dr Phil Smith, medical director of the Biocontainment Unit at Nebraska Medicine, said in a hospital release.

"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 Ebola disease develops."

The current outbreak that hit the globe with a two-year-old Guinean boy in December 2013 has already infected 20,206 people and killed 7,905 of them.