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

As the World Health Organization announced its strategic plan for combating 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.