The decision came after a rare, two-day closed-door session of the UN health body's emergency committee, which urged exit screening of all people flying out of affected countries, where nearly 1,000 people have died.
The WHO stopped short of calling for global travel restrictions, urging airlines to take strict precautions but to continue flying to the area.
And it called on countries and airports around the globe to be prepared to "detect, investigate and manage" Ebola cases if they should arise.
WHO director-general Dr Margaret Chan appealed for greater help for the countries worst hit by the "largest, most severe and most complex outbreak in the nearly four-decade history of this disease".
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"I am declaring the current outbreak a public health emergency of international concern," Chan said, stressing the "serious and unusual nature of the outbreak".
Defining the epidemic a public health emergency of international concern -- a label only used twice before, during the H1N1 swine flu pandemic in 2009 and last May for the reemergence of polio in a number of countries -- "alerts the world to the need for high vigilance," she said.
A patient in Uganda tested negative for Ebola as fears were sparked of a spread to east Africa.
Meanwhile Benin -- to the east of the main affected countries -- awaited test results from two patients with Ebola-like symptoms.
Ebola has claimed at least 932 lives and infected more than 1,700 people since breaking out in Guinea earlier this year, according to the WHO.
States of emergency were in effect in Liberia, Guinea and Sierra Leone -- something WHO said was a first necessary step to bringing the outbreak under control.
"This outbreak really underscores the importance of having strong health systems," he said.
Despite the new measures, Fukuda acknowledged "the likelihood is that it will get worse before it gets better," adding that WHO was bracing to deal with the outbreak "at some level for some number of months."
Soldiers in Liberia's Grand Cape Mount province -- one of the worst-affected areas -- set up road blocks to limit travel to the capital Monrovia, as bodies reportedly lay unburied in the city's streets.