The announcement has followed a two-day meeting of the organisation’s emergency committee.
The Ebola outbreak constitutes "a public health risk to other states," a WHO emergency committee said in a statement.
"The possible consequences of further international spread are particularly serious in view of the virulence of the virus, the intensive community and health facility transmission patterns, and the weak health systems in the currently affected and most at-risk countries," the committee said.
The WHO also said a general ban on international travel and trade was not necessary.
Over 900 people have died and more than 1,700 have been infected since the outbreak of the Ebola virus disease in West Africa in February, the World Health Organisation (WHO) said on Wednesday.
Between August 2 and 4 alone, 108 new Ebola virus disease (EVD) cases were registered, and 45 people died. These figures included not only laboratory confirmed cases but also suspicious ones.
The largest number of deaths was reported in Guinea, where 363 people have died and 495 have been infected. In Sierra Leone, 286 people have died; in Liberia, 282.
There is no licensed treatment or vaccine for Ebola and the death rate has been about 50%.