The death toll from the Ebola virus outbreak has risen to 4,447, with most victims in West Africa, the World Health Organization (WHO) estimates.
There could be up to 10,000 new cases per week within two months if global efforts are not stepped up, WHO official Bruce Aylward warned.
But the rate of new cases of infection at some of the "epicentre" areas has slowed down, he added.
Sierra Leone, Liberia and Guinea have been hardest hit by the outbreak.
There have been 8,914 cases overall, including the fatal cases.
The WHO says it expects this number to top 9,000 by the end of the week.
The WHO estimates its figures by taking the numbers of confirmed cases and multiplying them - from Guinea by 1.5, from Sierra Leone by 2 and from Liberia by 2.5 - to account for underreporting.
Mr Aylward told reporters in Geneva that the WHO was concerned to see the virus continue to spread in the capitals of the three worst-affected countries.
He said the death rate in the current Ebola outbreak was 70%, describing it as a "high mortality disease".
He said 95% of cases were still limited to areas in the "historic epicentre" of the outbreak, where the rate of new infections appeared to be slowing.
However, he stressed that it would be too early to read this as success.
The latest WHO projections suggested that there could be between 5,000 and 10,000 cases a week by December.
"It could be higher, it could be lower but it's going to be in that ball park," he said.
"In certain areas we're seeing disease coming down but that doesn't mean they're going to go to zero."
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