Vaccine manufacturers have agreed to help create a global stockpile of human vaccine for H5N1 bird flu, the head of the World Health Organization announced Wednesday.

The British firm GlaxoSmithKline said it will contribute enough vaccine for 25 million people to WHO over three years. French drug maker Sanofi-Aventis, American firm Baxter International Inc., and Omnivest of Hungary also said they were willing to make some of their H5N1 vaccine available.

"This is another significant step towards creating a global resource to help the world and especially to help developing countries in case of a major outbreak of H5N1 avian influenza," said Dr. Margaret Chan, WHO director general, in a statement.

"WHO welcomes this contribution from the vaccines industry and is also working with countries to develop capacity for the production of influenza vaccines."

The H5N1 strain of bird flu mainly affects birds but has resulted in 190 human deaths since 2003, according to WHO. Scientists are watching for signs it has mutated into a form that transmits easily between humans, which could lead to a global pandemic.

More work is needed to clarify under what conditions the stockpile will be used, said the agency, which intends to use the supply to try to stamp out cases at the source of a pandemic, and to help developing countries protect health-care workers, police and military during a pandemic.

Earlier this spring, the agency said about 40 million to 60 million doses might be needed.

It is expected that the companies will announce how many doses they are contributing to the stockpile.

GlaxoSmithKline said its vaccine would offer "a degree of protection" in the four to six months between the time a vaccine tailored to the actual pandemic strain is identified and the time production begins.

The product's shelf life is about three to five years.