The UN health agency issued a strategic plan to combat the oubreak in four West African nations where it said the actual number of cases could already be two to four times higher than the reported 3,069.
The death toll now stands at 1,552.
A statement issued by the WHO said: "This roadmap assumes that in many areas of intense transmission the actual number of cases may be two-four fold higher than that currently reported. It acknowledges the aggregate case load of Ebola Virus Disease could exceed 20,000 over the course of this emergency."
The deadly outbreak that began in Guinea in March and has spread to neighbouring Liberia and Sierra Leone as well as to Nigeria requires a massive and coordinated international response, the WHO said.
It added: "Response activities must be adapted in areas of very intense transmission and particular attention must be given to stopping transmission in capital cities and major ports, thereby facilitating the larger response and relief effort."
The virus is still being spread in a "substantial number of localities", aggravating fragile social and economic conditions and has already killed an unprecedented number of health workers, the agency said.
A wider UN-led plan being launched by the end of September is "expected to underpin support for the increasingly acute problems associated with food security, protection, water, sanitation and hygiene, primary and secondary health care and education, as well as the longer-term recovery effort that will be needed," the WHO said.
Meanwhile, a doctor in Nigeria's oil industry hub of Port Harcourt has died from Ebola after he was infected by man linked to the first case in Africa's most populous country.
The Health Ministry said the doctor had treated a primary contact of Patrick Sawyer, the Liberian who brought Ebola to Lagos.