"Our challenge now is to quarantine the area to successfully break the transmission," he said, referring to the Voinjama area.
Many people may be hiding, but many are also finally beginning to seek treatment as more centers open up.
Beds in such centers are filling up faster than they can be provided, evidence that the outbreak in West Africa is far more severe than the numbers show, Gregory Hartl, a spokesman for World Health Organization in Geneva, said.
There are 40 beds at a treatment center that Doctors Without Borders known as MSF recently took over in one quarantined county in Liberia but 137 people have flocked there, packing the hallways until they can be sorted into those who are infected and those are not, said MSF's international president, Joanne Liu.
"It's absolutely dangerous," said Liu, who recently returned from Guinea, Liberia and Sierra Leone. "With the massive influx of patients that we had over the last few days, we're not able to keep zones of patients anymore. Everybody is mixed."
The charity will have 300 beds available once several dozen open in the coming days, and Liu said it cannot possibly provide any more.
Hartl, of the World Health Organization, noted that an 80-bed treatment center opened in Liberia's capital in recent days filled up immediately. The next day, dozens more people showed up to be treated.
Liu likened the situation to a state of war because the "frontline" was always moving and unpredictable. She said the outbreak could last six more months.
"We're running behind a train that is going forward," Liu told reporters in Geneva on Friday. "And it literally is faster than what we're bringing in terms of a response."
The U.N. health agency warned Thursday that the official counts of 1,069 dead and 1,975 infected may still "vastly underestimate the magnitude of the outbreak."
It said extraordinary measures are needed "on a massive scale to contain the outbreak in settings characterized by extreme poverty, dysfunctional health systems, a severe shortage of doctors, and rampant fear."
Ebola causes a high fever, bleeding and vomiting. It has no cure and no licensed treatment and has been fatal in at least 50 percent of the cases, health experts say. The disease is usually found in eastern or central Africa, typically in rural, isolated communities, where outbreaks tend to be "self-limiting," Hartl said.
By contrast, the current outbreak spread quickly to cities and the capitals of Guinea, Liberia and Sierra Leone, making it difficult to stop.