A doctor with the World Health Organization in Sierra Leone has tested positive for the Ebola virus.

The physician is in stable condition in Freetown and will be relocated shortly, the UN health agency said in a statement Monday.

To maintain patient confidentiality, WHO spokeswoman Nyka Alexander in Freetown said the agency won't provide details on nationality or where the person is going for treatment.

The doctor was working at the Ebola treatment centre in the Kenema Government Hospital, which is run by Sierra Leone's health ministry, CBC reported.

The physician had symptoms on Saturday that were confirmed yesterday as Ebola, and the evacuation is scheduled for today, Alexander said.

To ensure the safety of those inside the hospital, WHO recommended to the health ministry that new admissions be limited while an investigation is launched.

"Every time a health-care worker is infected, it's very worrisome," Alexander said.

"What we're seeing from some of the investigations is it's not always an infection within the facility. You can't be sure that the infection didn't happen within the community, for example, or outside of the ward."

Given there are so many questions, investigations are important to try to figure out what happened and to make nobody else is at risk, Alexander added.

Meanwhile, a fourth American who contracted Ebola in West Africa is expected to arrive in the U.S. for care Tuesday and will be treated at an Atlanta hospital where two other aid workers successfully recovered from the disease, Emory University Hospital said Monday. The hospital cited confidentiality in not releasing more information about the person.

A total of 152 health-care workers have been infected and 79 have died.