By Stephanie Nebehay and Tim Cocks

GENEVA/LAGOS (Reuters) - The Ebola epidemic in West Africa could infect more than 20,000 people, the U.N. health agency said on Thursday, warning that an international effort costing almost half a billion dollars is needed to overcome the outbreak.

As the World Health Organisation (WHO) announced its strategic plan for combatting the virus, GlaxoSmithKline said an experimental Ebola vaccine is being fast-tracked into human studies and it plans to produce up to 10,000 doses for emergency deployment if the results are good.

The WHO estimates it will take six to nine months to halt the Ebola epidemic in West Africa, while Nigeria said on Thursday that a doctor involved in treating the Liberian-American who brought the disease to the country had died in Port Harcourt, Africa's largest energy hub, although the cause had yet to be confirmed.

So far 3,069 cases have been reported in the outbreak but the WHO said the actual number in Guinea, Sierra Leone, Liberia and Nigeria could already be two to four times higher.

"This is not a West Africa issue. This is a global health security issue," Bruce Aylward, the WHO's Assistant Director-General for Polio, Emergencies and Country Collaboration, told reporters in Geneva.

With a fatality rate of 52 percent, the death toll stood at 1,552 as of Aug. 26. That is nearly as high as the total from all recorded outbreaks since Ebola was discovered in what is now Democratic Republic of Congo in 1976.

The figures do not include deaths from a separate Ebola outbreak announced at the weekend in Congo, which has been identified as a different strain of the virus.

Aylward said tackling the epidemic would cost an estimated $490 million, involving thousands of local staff and 750 international experts. "It is a big operation. We are talking (about) well over 12,000 people operating over multiple geographies and high-risk circumstances. It is an expensive operation," he said.

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The operation marks a major ramping up of the response by the WHO, which had been accused by some aid agencies of reacting too slowly to the outbreak.

A wider United Nations -led plan being launched by the end of September is expected to provide support for the secondary effects of the outbreak on food security, water, sanitation, primary and secondary healthcare and education, the WHO said.

EXPERIMENTAL DRUGS

Early this month, the WHO called the current Ebola outbreak an "international health emergency". Concerns that the disease could spread beyond West Africa have led to the use of drugs still under development for the treatment of a handful of cases.

Two American health workers, who contracted Ebola while treating patients in Liberia, received an experimental therapy called ZMapp, a cocktail of antibodies made by tiny California biotech Mapp Biopharmaceutical. They recovered and were released from hospital last week.

The virus has already killed an unprecedented number of health workers and is still being spread in a many places, the agency said. About 40 percent of the cases have occurred within the past 21 days, WHO statistics showed.

Previous Ebola outbreaks have mainly occurred in isolated areas of Central Africa. However the current epidemic has spread to three West African capitals and Lagos, Africa's biggest city. The WHO said special attention would need to be given to stopping transmission in capital cities and major ports.

Authorities in Nigeria announced the death in Port Harcourt, the oil industry hub of Africa's largest crude exporter, of one of the doctors who treated Patrick Sawyer, a U.S. citizen who died in Lagos after flying in from Liberia last month.

Health Ministry spokesman Dan Nwomeh wrote in his Twitter feed that 70 people who had been in contact with Sawyer were now under observation in the town. Aylward said the deceased doctor had not yet been confirmed as being infected with Ebola.

According to new figures released on Thursday, Nigeria has recorded 17 cases, including six deaths, from Ebola, since Sawyer collapsed upon arrival at Lagos airport in late July.

AID EFFORT CHOKED OFF

The Lagos case contributed to the decision by a number of airlines to halt services to Ebola-affected countries. Air France said on Wednesday it had suspended flights to Sierra Leone on the advice of the French government.

The WHO has advised against travel bans and border closures, which some countries in the region have also implemented, saying they risked creating food and supply shortages.

"We assume current airline limitations will stop within the next couple of weeks. This is absolutely vital," Aylward said. "Right now the aid effort risks being choked off."

West African health ministers meeting in Ghana on Thursday echoed the WHO's concerns and called for the reopening of borders and an end to flight bans.