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By Laura MacInnis

GENEVA, July 10 (Reuters) - Only a fraction of the nearly $1.9 billion pledged by international donors in January to help the developing world prepare for a bird flu pandemic has been paid out so far, the United Nations said on Monday.

In a joint report with the World Bank, the U.N. bird flu coordinator David Nabarro said donor countries had allocated $1.15 billion for bird flu aid from their budgets by the end of April, but had transferred just $331 million to recipients.

Although the virus affects mostly wild birds, experts fear it may change into a form that can be easily transmitted among humans, sweeping the world and killing millions within weeks or months.

"Our concern must tighten as we move into the colder months in the northern hemisphere [when the virus spreads more rapidly]," he said after a meeting of the U.N. Economic and Social Council on global preparedness for a bird flu pandemic.

Much of the shortfall reflected slow payments from the World Bank and Asian Development Bank, lenders which together accounted for $969 million of the pledges made in Beijing but had released just $4 million by the end of April.

The United States had set aside the $334 million it pledged but disbursed only about $71 million, according to the report. The European Commission had locked in $178 million -- more than its $124 million pledge -- but had not distributed any funds.

However, Japan had disbursed nearly $158 million, exceeding the $155 million it promised.

The donor funds are designated for upgrading veterinary systems, vaccination drives and education about animal hygiene.

Nabarro said more money was needed to ensure poorer countries in Africa, Latin America and elsewhere were ready for a resurgence of the bird flu threat later this year.

He said Cameroon, Burkina Faso and Egypt were among those short of funds for pandemic preparation.

The H5N1 bird flu strain has caused outbreaks in more than 48 countries and territories since re-emerging in Asia three years ago, and has killed more than 130 people.

"It is certainly moving into more and more countries with a speed that is, for me and my colleagues, a continuing concern," Nabarro said.

He added the African Union would host a major meeting in Mali in December to assess global needs.

"This is a virus that has really quite extraordinary damaging potential, and that is why we are concerned about it," he said.