DAVOS, Switzerland, Jan. 31 (AFP) – Microsoft Chairman Bill Gates, the world's richest man, promised $10 billion (7.2 billion euros) to develop ''miracle'' vaccines for the world's poorest nations.

Gates announced at the World Economic Forum that the money will come over the next decade from the foundation he runs with his wife Melinda, and that vaccines will become the charity's top priority.

The Bill and Melinda Gates Foundation has now committed more than $25 billion to various health projects, especially targeting AIDS and polio and other diseases that hit poor countries.

''We must make this the decade of vaccines,'' Gates said in Davos. ''Vaccines already save and improve millions of lives in developing countries.

''Innovation will make it possible to save more children than ever before.''

Increased vaccination could save more than eight million children by 2020, he added, but called on governments and the private sector to do more.

''Increased investment in vaccines by governments and the private sector could help developing countries dramatically reduce child mortality by the end of the decade,'' Gates said in a statement.

Melinda Gates added: ''Vaccines are a miracle – with just a few doses, they can prevent deadly diseases for a lifetime.

''We've made vaccines our number-one priority at the Gates Foundation because we've seen first hand their incredible impact on children's lives,'' she added.

UN agencies and campaign groups welcomed Gates stunning announcement.

''The Gates Foundation's commitment to vaccines is unprecedented, but needs to be matched by unprecedented action,'' said World Health Organization director general Margaret Chan.

''It's absolutely crucial that both governments and the private sector step up efforts to provide life-saving vaccines to children who need them most.''

One, the activist group set up by rock singer Bono to fight against poverty and preventable disease, said Gates's money ''is a testament to their belief in catalyzing transformative change.''

But it also said that western governments and institutions must ''accelerate their own investments in effective organizations.''

Gates began working full time at the Foundation after stepping down from heading up Microsoft business operations in July, 2008.