The latest Ebola outbreak is the largest and longest ever recorded for the disease, which has a death rate of about 50 per cent and has so far killed at least 961 people, according to the UN health agency.
It emerged in Guinea in March and has since spread to Sierra Leone, Liberia and Nigeria.
"Countries affected to date simply do not have the capacity to manage an outbreak of this size and complexity on their own," WHO chief Dr. Margaret Chan said.
"I urge the international community to provide this support on the most urgent basis possible."
She added that the world's "collective health security" depends on curbing the spread of the killer virus in West Africa, even as she acknowledged that many countries would probably not have any Ebola cases.
The Nigerian government declared containing the Ebola virus in Africa's most populous country a national emergency yesterday, after two Ebola patients died and the health ministry said seven other cases were confirmed.
President Goodluck Jonathan approved spending £7 million to fight the disease.