(Adds background on Ebola outbreak, missionaries affected)
Sept 2 (Reuters) - An American doctor working in Liberia has tested positive for the Ebola virus after working with obstetrics patients at a missionary hospital in Monrovia, the Christian organization SIM USA said on Tuesday.
The North Carolina-based group did not identify the doctor, who had not been treating the Ebola patients hospitalized in isolation on the missionary's sprawling campus.
It is not yet known how the physician contracted the often deadly virus, the organization said.
"The doctor is doing well and is in good spirits," SIM USA said in a statement.
The worst Ebola outbreak in history has infected more than 3,000 people and killed some 1,550 since it was first detected early this year in West Africa, according to the World Health Organization.
The crisis appears to be worsening, U.S. health officials said on Tuesday, predicting the number of Ebola cases will keep rising in the next two weeks.
Missionaries from SIM USA, based in Charlotte, have been on the front lines of the epidemic. Another of its U.S. aid workers, Nancy Writebol, contracted the virus in July, also while working in Liberia.
She went home two weeks ago after receiving care in an isolation unit at Emory University Hospital in Atlanta. She was also one of a few patients to receive an experimental treatment called ZMapp, although doctors at Emory said they could not determine whether it had helped her recovery. She has since been resting in an undisclosed location.
The U.S. doctor involved in the new Ebola case immediately isolated himself when symptoms began, according to a SIM USA news release.
He was later transferred to the Ebola isolation unit on the group's 136-acre (55-hectare) campus in Monrovia.
The missionary group could not immediately be reached to discuss the doctor's condition and plans for his medical treatment.
"My heart was deeply saddened, but my faith was not shaken, when I learned another of our missionary doctors contracted Ebola," said Bruce Johnson, president of SIM USA said in a statement. (Reporting by Letitia Stein in Tampa, Fla., David Morgan in Washington and Bangalore Newsroom; Editing by Diane Craft and Eric Beech)