The World Health Organisation (WHO) says the Ebola outbreak in West Africa eventually could exceed 20,000 cases, more than six times as many as doctors know about now.
A new plan to stop Ebola by the UN health agency also assumes that in many hard-hit areas, the actual number of cases may be two to four times higher than is currently reported.
The agency on Thursday published new figures saying that 1,552 people have died from the killer virus from among the 3,069 cases reported so far in Liberia, Sierra Leone, Guinea and Nigeria.
The agency says it needs as much as half a billion dollars to contain the outbreak.
At least 40 percent of the cases have been in just the last three weeks, the UN health agency said, adding that "the outbreak continues to accelerate".
It comes as Nigeria on Thursday said that a doctor had died from Ebola in the southeastern oil city of Port Harcourt in the first case of the deadly virus outside the financial hub, Lagos.
"The cases are increasing. I wish I did not have to say this, but it is going to get worse before it gets better," Tom Frieden, the director of the Centres for Disease Control and Prevention, told a news conference in Monrovia on Wednesday.
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"The world has never seen an outbreak of Ebola like this. Consequently, not only are the numbers large, but we know there are many more cases than has been diagnosed and reported," Frieden said on Wednesday.
Liberia has been hardest-hit by the epidemic now raging through West Africa, with 624 deaths and 1,082 cases since the start of the year.
The WHO said the "unprecedented" outbreak has killed 1,427 people this year, while about 2,615 people have been infected with the disease.
But the WHO also believes its count is probably far too low, due in part to community resistance to outside medical staff and a lack of access to infected areas.
Health ministers from West African nations hit by Ebola gathered in the Ghanaian capital, Accra, on Thursday to discuss responses to the epidemic.
Also on Thursday, it is expected that US health officials will announce a human study of an Ebola vaccine made by GlaxoSmithKline will begin in the coming weeks.
On Wednesday, airlines ended more services to African countries hit by the Ebola outbreak that has left more than 1,400 people dead as health ministers prepared for a summit to deal with the crisis.
Air France agreed to a government request on Wednesday for a "temporary suspension" of services to Sierra Leone, leaving its capital Freetown and Monrovia in neighbouring Liberia with just one regular service, from Royal Air Morocco.
The decision came a day after British Airways said it was suspending flights to Liberia and Sierra Leone until next year due to Ebola concerns.
Authorities in the worst-hit nations are scrambling to contain the most serious outbreak of the lethal tropical virus in history.
Brussels Airlines has cancelled several services in Liberia, Sierra Leone and Guinea. The carrier said it would decide on its future schedule this weekend.
The company committed to providing three separate flights to Freetown, Monrovia and Conakry this week in response to passenger demand and to deliver 40 tonnes of medical supplies from the UN.