The scale of the Ebola crisis has been "vastly underestimated", according to the United Nations' health agency,
The World Health Organization said "extraordinary measures" were needed to tackle the outbreak, which has killed 1,069 people.
The WHO added its staff had seen evidence that the numbers of reported cases and deaths do not reflect the scale of the crisis.
The current outbreak began in Guinea in February.
It has since spread to Sierra Leone, Liberia and Nigeria.
About 60 per cent of those infected in this outbreak have died.
The WHO said the outbreak was expected to continue "for some time," adding: "WHO is co-ordinating a massive scaling up of the international response."
Ebola causes severe dehydration through vomiting, bleeding and diarrhoea.
There is no cure and no vaccine to stop infection.
