The Ebola response in Liberia, the country worst hit by the outbreak, will focus on community-level care units since new treatment centers are unlikely to be ready for weeks or months, World Health Organization Assistant Director General Bruce Aylward said on Tuesday.

"The absolute first priority is to establish enough capacity to rapidly isolate the cases so that they are not infecting others. We need Ebola treatment centers to do that, very very quickly, but they take time to build, as you've seen," he said.

"It takes weeks, if not months, to get these facilities up and running. We have firm commitments for more than 500 additional beds in Liberia and we think we will hear announcements that will take that even further over the coming weeks."

The WHO still has a goal to "bend the curve" in total Ebola case numbers across West Africa within three months, but some areas may be free of the disease sooner, he said.

"You definitely want to get Nigeria and Senegal obviously done quickly," Aylward said. "In some capitals - Freetown, Conakry - we should be able to get those free in the near term. Guinea should be able to get most of the country free in the very near term as well."

In Sierra Leone and Liberia the disease is more entrenched over bigger geographic areas and the Liberian capital Monrovia was a "particular challenge", he said.

The number of cases has shrunk to one single confirmed Ebola patient in Senegal, after two suspected cases were ruled out, and remained steady at 21 cases in Nigeria, he said.

"I cannot say Senegal is safe. Remember, if a country has Ebola, the incubation period is about 21 days. I like to see at least two incubation periods without any cases to be absolutely sure. So that would take us way out into October. Never declare victory over this virus."

Guinea, where the outbreak originated last December, has had 936 cases, Sierra Leone 1,602 and Liberia 2,407, he said.