Nearly 12 million Africans deemed at highest risk from yellow fever will be vaccinated next week against the virus, which can cause explosive epidemics in cities, the World Health Organization said on Tuesday.

The vaccination drive will span three countries -- Benin, Liberia and Sierra Leone -- starting on Monday and take about a week, the WHO said in a statement.

Yellow fever is a viral haemorrhagic fever which can cause devastating epidemics, particularly in urban centers, Rosamund Lewis, project leader in WHO's yellow fever initiative, told a news briefing.

Vaccination is the best way to prevent yellow fever, which is hard to diagnose in early stages and for which there is no specific treatment, according to the United Nations agency.

The Global Alliance for Vaccines and Immunization (GAVI) has contributed $103 million toward a U.N. vaccine stockpile for yellow fever, the WHO statement said.

France's Sanofi-Aventis, Brazil's Biomanguinhos and the Institut Pasteur de Dakar are the three suppliers of yellow fever vaccine pre-qualified by the WHO for U.N.-wide use.

Yellow fever infects 206,000 people a year and kills an estimated 30,000, mainly in tropical parts of Africa and the Americas where it has never been wiped out.

Some 160 million people could still be at risk in Africa if further funding is not secured, the WHO said. Vaccination campaigns must still be carried out in five more countries -- Central African Republic, Ghana, Guinea, Ivory Coast and Nigeria -- to shield those most at risk.

Most countries require yellow fever vaccination for travel to and from endemic countries to avoid having an outbreak spread elsewhere. Yellow fever is reappearing in countries that have not reported cases in many years, said Fenella Avokey of the WHO's African regional office.

Cases of yellow fever have also appeared in Latin America, including Argentina, Paraguay and Brazil, where the virus had never been seen before, according to Lewis.

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