WASHINGTON, June 21 (Reuters) - The United States has pledged more than a quarter of all the funds being used to prepare the world for an influenza pandemic, but is still having trouble identifying which countries need the most help, according to a report released on Thursday.

U.S. agencies have committed about $377 million to improve global preparedness for avian and pandemic influenza, said the report by the Government Accountability Office, the nonpartisan investigative arm of Congress.

"This amounted to about 27 percent of the $1.4 billion committed by all donors combined; exceeded the amounts other individual donors, including the World Bank, the Asian Development Bank, and Japan, had committed; and was also greater than combined commitments by the European Commission and European Union member countries," the GAO report reads.

"The U.S. Agency for International Development and the Department of Health and Human Services have provided most of these funds for a range of efforts, including stockpiles of protective equipment and training foreign health professionals in outbreak response," the report added.

Health experts almost universally agree that the world is overdue for some sort of pandemic of influenza. Flu pandemics hit three times in the past century, killing millions, and it has been 40 years since the last one.

The H5N1 avian flu virus currently circulating among birds in Asia, parts of Europe and Africa is the No. 1 suspect. It is responsible for the death or destruction of an estimated 200 million birds. It rarely affects humans but has infected 313 people and killed 191 since 2003.

A few small mutations could give it the ability to pass easily from one person to another, sparking a pandemic.

Health experts also agree that almost no one is ready for this, and a pandemic would not only kill millions but would devastate economies, disrupt industries from tourism to retail and bring trade to a near standstill for months.

The GAO report said the Homeland Security Council had designated 20 priority countries for U.S. assistance, with the top three being Indonesia, Nigeria, and Egypt.

"USAID ... emphasized that in the coming months the agency will be focusing in particular on developing more effective approaches to controlling the spread of H5N1 in small-scale "e;backyard farms"e; where high-risk agricultural practices are common," the report reads.

Most of the people infected with H5N1 have handled infected poultry, usually from small family flocks.

"Efforts to assemble more comprehensive information are under way, but will take time to produce results," the report added.

It said the U.S. Centers for Disease Control and Prevention was trying to develop a scorecard to systematically assess which countries were ready and where help was needed.