The U.N. agency issued a statement detailing why the outbreak in West Africa had been underestimated, following criticism that it had moved too slowly to contain the killer virus, now spreading out of control.

Independent experts raised similar concerns a month ago that the contagion could be worse than reported because suspicious local inhabitants are chasing away health workers and shunning treatment.

More than 1,300 people have died from the disease and many experts do not expect the epidemic to be brought under control this year.

Under-reporting of cases is a problem especially in Liberia and Sierra Leone. The WHO said it was now working with Medecins Sans Frontières (MSF) and the U.S. Centers for Disease Control and Prevention to produce "more realistic estimates".

The head of MSF, which has urged the WHO to do more, told Reuters in an interview on Thursday that the fight against Ebola was being undermined by a lack of international leadership and emergency management skills.

The stigma surrounding Ebola poses a serious obstacle to efforts to calibrate the outbreak in Liberia, Sierra Leone, Guinea and Nigeria, which has claimed far more victims than any other episode of the disease that was first discovered nearly 40 years ago in the forests of central Africa.

"As Ebola has no cure, some believe infected loved ones will be more comfortable dying at home," the WHO statement said.

"Others deny that a patient has Ebola and believe that care in an isolation ward – viewed as an incubator of the disease – will lead to infection and certain death. Most fear the stigma and social rejection that come to patients and families when a diagnosis of Ebola is confirmed."

Corpses are often buried without official notification, the WHO said, while an additional problem is the existence of numerous "shadow zones", or rural villages where there are rumours of cases and deaths that cannot be investigated because of community resistance or lack of staff and transport.

In other cases, where treatment is available, health centres are being immediately overwhelmed with patients, suggesting there is an invisible caseload of patients that is not on the radar of the official surveillance systems.

STRATEGY PLAN

The WHO said it had drawn up a draft strategy plan to combat Ebola in West Africa over the next six to nine months, implying that it does not expect to halt the epidemic before the end of the year.

"WHO is working on an Ebola road map document; it's really an operational document how to fight Ebola," WHO spokeswoman Fadela Chaib said at a news briefing. "It details the strategy for WHO and health partners for six to nine months to come."

Chaib, asked whether the timeline meant that the United Nations health agency expected the epidemic now raging in Guinea, Liberia and Sierra Leone to continue into 2015, said: "Frankly, no one knows when this outbreak of Ebola will end."

Ebola will be declared over in a country if two incubation periods, or 42 days in total, have passed without any confirmed case, she said. Nigeria is the fourth country with known cases.

"So with the evolving situation, with more cases reported, including in the three hot places - Guinea, Sierra Leone, and Liberia - the situation is not yet over," Chaib said.

"So this is a planning document for six to nine months that we will certainly revisit when we have new developments."

The WHO expects to issue details of the plan early next week, she said.

In a sign of spreading international alarm, Senegal, West Africa's humanitarian hub, said it had blocked a regional U.N. aid plane from landing and was banning all further flights to and from countries affected by Ebola, potentially hampering the emergency response to the epidemic.

(Reporting by Stephanie Nebehay in Geneva, Ben Hirschler in London and Emma Farge in Dakar; Editing by Mark Heinrich)