The United Nations health agency says an influenza outbreak among migratory birds in Northwestern China is far worse than the government had reported.

World Health Organization experts who recently visited China's northwestern province of Qinghai told reporters that five times as many birds have died of the avian flu there as originally reported.

The agency estimates five-thousand wild birds died sometime during the spring on an island in Qinghai. In late May, the Chinese government put the number of dead at only one-thousand.

Dr. Julie Hall, the WHO's coordinator for communicable diseases in China, said this is the first time that migratory birds have been found to die from the avian flu in such numbers.

She urged Chinese authorities to conduct immediate tests for the H5N1 virus, before the birds migrate to neighboring countries.

"So we feel that there's a window of opportunity here to study these birds, to understand which birds can carry the virus and which birds cannot. But this window of opportunity is very narrow, two to three months," she said.

Experts are worried that birds that carry the virus may spread it to other animals as they move from place to place.

Tens of millions of domestic fowl have already died or been killed in Southeast Asia to prevent the spread of the virus. Avian influenza can be transmitted to humans through close contact with infected birds. Since 2003, 54 people have died in Vietnam, Thailand, and Cambodia from the disease.

Health experts are worried that the virus may mutate to a form easily transmitted between humans, possibly sparking a global flu pandemic that could kill millions.

China has been criticized in the past for failing to report outbreaks of contagious diseases. In 2003, authorities played down the outbreak of Severe Acute Respiratory Syndrome before it spread around the world, killing 800 people.

Last week, Beijing denied that the government had encouraged farmers to vaccinate chickens against bird flu with an anti-viral drug meant for humans. Experts say such a practice would diminish the drugs' ability to fight infection.