New figures showed the massive scale of the crisis, which the WHO said indicated a "rapid increase still in the intensity of transmission" that could cost at least $490 million (370 million euros) to tackle.
Bruce Aylward, the WHO's head of emergency programmes, said it could take six to nine months to bring Ebola under control, by which time the number of infections could have passed 20,000.
"That's not saying we expect 20,000, that's not saying we would accept, more importantly, 20,000 cases," he told reporters in Geneva, calling the situation a "global health security issue".
"But we have got to have a system that is robust enough to deal with... a very bad case scenario."
As of August 26, 1,552 people had been confirmed dead from Ebola in four countries -- Sierra Leone, Liberia, Guinea and Nigeria -- while 3,062 had been infected.
Liberia was the worst affected with 694 deaths; 422 people have died in Sierra Leone; and 430 in Guinea, where the virus emerged at the start of the year. Nigeria has now recorded six deaths.
AFP