WASHINGTON, Nov 28 (Reuters) - As much as $1.3 billion more is needed to fight bird flu, with more than $500 million of that going to Africa, World Bank and U.N. experts said on Tuesday.

This is on top of the $1.9 billion pledged at a World Bank conference in Beijing last January, said World Bank Economic Adviser Olga Jonas, who will present her official estimates to a meeting of bird flu experts that begins next Wednesday in Bamako, Mali.

"We foresaw only a very small amount that would be needed in Africa," Jonas said in an interview.

But since January, H5N1 avian influenza has spread out of Asia, across Europe and into Africa. Now more than 50 countries have battled the virus, which mostly affects birds but which has infected 258 people and killed 153 since 2003.

Among them are some of the poorest countries in Africa -- Uganda, Niger, Nigeria, Ivory Coast, Burkina Faso and Djibouti, as well as Egypt.

U.N. bird flu coordinator Dr. David Nabarro said the money pledged worldwide in January was being spent rapidly but said much more needs to be done to prepare for a pandemic, or to try to prevent one.

Jonas said globally, between $980 million and $1.3 billion is needed over the next two to three years to fight bird flu. The money would go for everything from rubber gloves and disinfectants to cash compensation to people whose birds are culled if H5N1 is detected.

Some money has gone to African countries, but $566 million more is needed, she said, quoting figures prepared for the 4th International Conference on Avian Influenza, sponsored by the European Union, European Commission and the African Union.

UNFORESEEN SEVERITY

Indonesia is now the nation worst hit by H5N1, with 30 of its 33 provinces affected. Out of 74 reported human cases of infection in Indonesia, 57 have died.

"In Indonesia, the problem is just very, very severe and the programs to deal with it are also going to require more resources than we foresaw," Jonas said.

Not only does the virus threaten poultry -- more than 200 million fowl have been slaughtered or died -- but experts fear it could cause a human pandemic.

Just a few genetic mutations could make H5N1 spread easily from person to person, sparking a global epidemic that might kill millions and ruin economies.

Jonas said more than $720 million of the $1.9 billion pledged in January has been disbursed. Nabarro has been trying to keep tabs on it.

"I am upbeat because I have seen fantastic work being done, but I remain personally quite scared because I have seen the way in which this virus is still knocking around and not going away," Nabarro said.

Nabarro said new veterinarians were being hired and trained, better transport systems being set up for animals and he had seen more effective reporting of outbreaks in people and animals in several countries.

"There have been massive communications programs underway to inform people about the dangers associated with close contact with sick birds," Nabarro added.

The United Nations, the World Health Organization and other groups have said every country must step up efforts to prevent more bird flu outbreaks by watching birds and other animals more closely, moving quickly to cull infected birds, vaccinating animals and developing plans to deal with a human pandemic