The US had pledged 629 million dollars last December during a conference in New Delhi that raised 2.7 billion dollars to fight against the avian flu, which the UN says could cause a global crisis.
"The United States is pledging an additional 320 million dollars in international assistance for avian and pandemic influenza," said Paula Dobriansky, US Under Secretary of State for Democracy and Global Affairs told the conference in the Red Sea resort of Sharm el-Sheikh.
Dobriansky said she hoped other countries would increase their aid and warned against complacency towards the disease, according to a text of her speech furnished by the US embassy in Cairo.
The H5N1 virus strain first emerged in Asia in 2003 and has since caused some 245 deaths in humans, with Indonesia and Vietnam among the worst hit countries, according to World Health Organisation figures.
Scientists fear that H5N1 will eventually mutate into a form that is much more easily transmissible between humans, triggering a global pandemic.
According to a World Bank report, even a mild flu pandemic might kill 1.4 million people worldwide, while the death toll from a severe global outbreak could reach 70 million.
The report also said a flu pandemic of moderate intensity could cut global gross domestic product by two percent, while a severe flu pandemic would slash global gross domestic product by nearly five percent, or more than three trillion dollars.