The World Health Organization, or WHO, said Friday that the presence of “shadow zones” -- regions in the Ebola-affected countries inaccessible to doctors and health workers -- and under-reporting of cases have undermined its efforts to estimate the magnitude of the outbreak.

“Many families hide infected loved ones in their homes. As Ebola has no cure, some believe infected loved ones will be more comfortable dying at home,” the WHO said, in a statement released Friday. “In rural villages, corpses are buried without notifying health officials and with no investigation of the cause of death. In some instances, epidemiologists have travelled to villages and counted the number of fresh graves as a crude indicator of suspected cases.”

The United Nations health agency added that a “lack of adequate staff and vehicles” has further contributed to the creation of numerous shadow zones where a number suspected Ebola cases could not be investigated.

“In parts of Liberia, a phenomenon is occurring that has never before been seen in an Ebola outbreak. As soon as a new treatment facility is opened, it is immediately filled with patients, many of whom were not previously identified. This phenomenon strongly suggests the existence of an invisible caseload of patients who are not being detected by the surveillance system,” WHO said.

The WHO also said that its epidemiologists in Sierra Leone and Liberia are working closely with other agencies, including Medecins Sans Frontieres, or MSF, and the US Centers for Disease Control and Prevention, or CDC, to produce “more realistic estimates” and devise strategies to combat the disease over the next six to nine months.  

“It means more doctors, Liberian doctors, more nurses, Liberian nurses, and more equipment," David Nabarro, a senior U.N. system coordinator for Ebola, said during his trip to Liberia, according to media reports. “But it also means, of course, more international staff,” he added.

1,427 people have died and 2,615 people have so far been infected by the virus since the outbreak was first identified in March, according to the latest figures released by WHO on Friday. Among the Ebola-hit nations in West Africa, Liberia, which has so far reported over 1,000 cases and over 600 deaths, is the worst affected.