In an assessment released Thursday, the U.N. agency said the epidemic "continues to accelerate" with more than 40 percent of reported cases occurring within just the last three weeks.
The outbreak, which began in February, has affected 3,069 people in four West Africa nations -- Guinea, Liberia, Nigeria and Sierra Leone -- and 1,552 have already died.
"In many areas of intense transmission the actual number of cases may be four fold higher than currently reported," the assessment said.
On Tuesday, the WHO said more than 120 health workers have been killed by the disease, and announced it was temporarily withdrawing from one of its centers in Sierra Leone after a worker became infected.
The WHO said it was it was launching a $490 million campaign to strengthen laboratory facilities and add staff with expertise in treating the Ebola Virus Disease, and called for the establishment of isolation centers and supervised burials of victims.
There is no vaccine for Ebola, but some experimental treatments have shown some signs of success in treating the often-fatal disease.