The WHO said more than 240 health care workers working in Guinea, Liberia, Nigeria, and Sierra Leone have developed the disease with "more than 120" succumbing to the epidemic.
The announcement came as Japan said it was ready to provide a Japanese-developed anti-influenza drug as potential treatment to fight the rapidly expanding Ebola outbreak.
Chief Cabinet Secretary Yoshihide Suga told reporters on Monday that Tokyo can offer the tablet favipiravir, developed by a subsidiary of Fujifilm, any time at the request of the WHO.
Suga said Japan is watching for the WHO's decision on further details over the use of untested drugs, AFP reported.
In case of an emergency, Japan may respond to individual requests before any further decision by the WHO, he said.
The WHO said earlier this month that it is ethical to use untested drugs on Ebola patients given the magnitude of the outbreak.
Developed by Toyama Chemical, a Fujifilm subsidiary, to treat novel and re-emerging influenza viruses, the drug was approved by the Japanese health ministry in March.