WHO is calling for a coordinated international response to stop the virus from reaching other parts of Africa.
More than 930 people have died from Ebola in West Africa so far.
A Spanish priest was flown home yesterday after he contracted the illness.
Fr. Miguel Pajares, who's 75, was taken off a plane on a stretcher by health workers in protective clothing.
He was carried in an isolation chamber for his journey from the Liberian capital to Madrid.
Two American missionaries are also ill with the virus.
Dr Kent Brantly's also being held in isolation in the US after being flown from Liberia where he picked up the illness working for the charity Samaritan's Purse.
Nancy Writebol, is also being held at the unit.
WHO Director-General Dr Margaret Chan said there had to be "a clear call for international solidarity" to stop the spread.
"Countries affected to date simply do not have the capacity to manage an outbreak of this size and complexity on their own.
"I urge the international community to provide this support on the most urgent basis possible."