"The Ebola outbreak that is ravaging parts of West Africa is the largest and most complex and most severe in the almost four-decade history of this disease," the WHO's director general Margaret Chan told a news conference in Geneva.
"The number of new patients is moving far faster than the capacity to manage them."
About 2,300 people in West Africa have died of Ebola virus infection.
The epidemic started in March and has also reached Nigeria and Senegal.
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