The number of new Ebola cases in three West African nations may jump to between 5,000 and 10,000 a week by Dec. 1 as the deadly viral infection spreads, the World Health Organization said.

The outbreak is still expanding geographically in Guinea, Sierra Leone and Liberia and accelerating in capital cities, Bruce Aylward, the WHO's assistant director general in charge of the Ebola response, said in a briefing with reporters in Geneva. There have been about 1,000 new cases a week for the past three to four weeks and the virus is killing at least 70 percent of those it infects, he said.

"Any sense that the great effort that's been kicked off over the last couple of months is already starting to see an impact, that would be really, really premature," Aylward said. "The virus is still moving geographically and still escalating in capitals, and that's what concerns me."