GENEVA — A month after declaring the Ebola outbreak in West Africa a global health emergency, the World Health Organization warned on Friday that the disease is still outpacing the international response to contain it.

“The Ebola outbreak that is ravaging parts of West Africa is the largest, most severe and most complex in the nearly four-decade history of this disease,” Margaret Chan, the health organization’s director general, said at a news conference. “The number of new cases is moving far faster than the capacity to treat them.”

So far, 4,784 Ebola cases have been reported and more than 2,400 people have died in the outbreak, which is concentrated in Guinea, Liberia and Sierra Leone, Ms. Chan said on Friday, citing the latest data available. But she made clear the figures were “an underestimate.”

A surge of 400 new cases in Liberia in the past week, double the number of new cases in the preceding week, was “a particular cause for concern,” the health organization said. Sierra Leone reported 200 new cases in the past week and a high rate of transmission in the capital, Freetown, the W.H.O. said. Nearly half the total number of infections in West Africa and just over half the deaths occurred in the last 21 days, it said.