UNITED NATIONS (AP) —

A missionary group says another American doctor has contracted the Ebola virus while working at a hospital in Liberia.

In a statement issued Tuesday, Serving In Mission said the unnamed doctor had been treating women in the obstetrics ward of a hospital in the Liberian capital.

The group doesn't know how the doctor contracted Ebola since the ward is separate from the area where Ebola patients are being treated.

Last month two Americans, including one from SIM, were evacuated to the United States for treatment after contracting Ebola in Liberia.

The two received an experimental drug known as ZMapp and recovered from Ebola. The manufacturer says it has run out of supplies of the drug and it will take months to produce more.

More than 1,500 people have died across West Africa from Ebola.