The World Health Organization 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 Ebola in four decades had now climbed to 1145 in the four afflicted West African countries - Guinea, Liberia, Nigeria and Sierra Leone.
"It is deteriorating faster, and moving faster, than we can respond to," Joanne Liu, the chief of Doctors without Borders, known by its French acronym MSF, told reporters in Geneva.
She added that it could take six months to get the upper hand.
The WHO said it was coordinating "a massive scaling up of the international response" to the epidemic.
"Staff at the outbreak sites see evidence that the numbers of reported cases and deaths vastly underestimate the magnitude of the outbreak," it said.
Outbreak could take 'six months to bring under control'
The warning comes as medical charity MSF says tje Ebola epidemic is moving faster than the authorities can handle and could take six months to bring under control.
Elhadj As Sy, the new head of the International Federation of Red Cross and Red Crescent Societies, painted a similarly bleak picture after he too returned from west Africa.
"In Sierra Leone, we've already lost some of the best doctors, including one of the best virologists of the country - and not only of the country, but of the region," he said.
As Sy added that he agreed with MSF's six-month timeline for bringing the outbreak under control.
The epidemic erupted in the forested zone straddling the borders of Guinea, Sierra Leone and Liberia earlier this year, and later spread to Nigeria.
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 on Saturday.
No cure or vaccine is currently available for Ebola, with the WHO authorising the use of largely untested treatments in efforts to combat the disease.
