WHO officials said a co-ordinated response was essential to stop and reverse the spread of the virus.
In Nigeria, one of the latest countries to be affected, President Goodluck Jonathan has declared a national state of emergency.
More than 960 people have died from Ebola in West Africa this year, two of them in Nigeria, the WHO says.
There were 68 new cases and 29 deaths reported over the course of two days this week, the United Nations health agency revealed on Friday.
Guinea: 367
Sierra Leone: 298
Liberia: 294
Nigeria: 2
It brings the total number of cases to 1,779.
There were 26 new cases in Sierra Leone and 38 in Liberia, but no new cases in Guinea, where the outbreak began.
The WHO said the outbreak was an “extraordinary event”.
“The possible consequences of further international spread are particularly serious in view of the virulence of the virus,” it said after the meeting.
