In a statement posted on its website, the organization said more than 240 cases of the virus have been recorded in Guinea and Liberia.
Most of those cases are in Guinea, where 136 people have died. Another 11 people have died in Liberia. The Liberia figure has been revised down because one death was already counted in Guinea's toll and tests showed another was not attributable to the virus.
Ebola causes a high fever and internal and external bleeding. There is no cure and no vaccine and it has a high fatality rate.
The West African outbreak is highly unusual as the disease is typically found only in central or eastern Africa.