The warning came a day after the World Health Organisation said the scale of the epidemic had been vastly underestimated and that "extraordinary measures" were needed to contain the killer disease.
The UN health agency said the death toll from the worst outbreak of the disease in four decades had now climbed to 1,069 in the four afflicted countries, Guinea, Liberia, Nigeria and Sierra Leone.
"It is deteriorating faster, and moving faster, than we can respond to," MSF (Doctors Without Borders) chief Joanne Liu told reporters in Geneva, saying it could take six months to get the upper hand.
WHO said yesterday it was coordinating "a massive scaling up of the international response" to the epidemic.
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"Staff at the outbreak sites see evidence that the numbers of reported cases and deaths vastly underestimate the magnitude of the outbreak," it said.
The latest epidemic erupted in the forested zone straddling the borders of Guinea, Sierra Leone and Liberia, and later spread to Nigeria.
WHO declared a global health emergency last week - far too late, according to MSF, which months ago warned that the outbreak was out of control.
"If we don't stabilise Liberia, we'll never stabilise the region," Liu said.
Concerns have also centred on the Nigerian cases, which are in Lagos, sub-Saharan Africa's largest city.
"Right now we have no past experience with in urban setting," said Liu.
As countries around the world stepped up measures to contain the disease, the International Olympics Committee said athletes from Ebola-hit countries had been barred from competing in pool events and combat sports at the Youth Olympics opening in China tomorrow.
No cure or vaccine is currently available for Ebola, which the WHO has declared a global public health emergency.