Experts fear that if the virus mutates to allow easy transmission among humans, it could create a global pandemic, killing millions of people. Recent studies, cited by WHO officials, show the disease would take three months for a possible pandemic to spread around the world.

Jia Youling, director of the Veterinary Bureau of the Agriculture Ministry, also confirmed that hundreds of cattle with foot-and-mouth disease have been slaughtered near Beijing since early May, and he insisted the case was handled properly despite the government's failure to announce it sooner.

The scarce information released about both disease outbreaks had fueled concern about a possible cover-up and rumors on Web sites that as many as 120 people had died of the avian flu.

No Fatalities Have Been Reported in China

More than 1,000 bar-headed geese, great black-headed gulls and other birds found this month in the western province of Qinghai died of the H5N1 strain of bird flu, Jia said.

"It is a rarity for such large-scale deaths to occur, whether in China or other parts of the world. We have never heard of such a thing," Jia said at a hastily called news conference.

Nevertheless, he said, "No person in Qinghai has been infected by the virus."

The regional death toll in Asia's latest bird flu outbreak stands at 54, but no fatalities have been reported in China. Vietnam suffered the most deaths with 38.

Could Create a Global Pandemic

The World Health Organization has warned that bird flu poses a great potential threat to humans if it evolves into a virus that can easily spread from person to person.

Experts fear that if the virus mutates to allow easy transmission among humans, it could create a global pandemic, killing millions of people.

Recent studies, cited by WHO officials, show the disease would take three months for a possible pandemic to spread around the world.

China initially reported 178 geese found dead in Qinghai Lake, a vast saltwater lake that is a major transit point for migratory birds, but raised the toll this week to 519. Jia did not explain why the number increased.

The deaths prompted the government to order all 3 million of the chickens, ducks and other poultry in Qinghai vaccinated. Nature reserves were closed to the public and farms near migration routes were told to watch for signs of disease.

Health experts worry that avian flu could be spread by birds whose migration routes cross Asia from Siberia through China and Southeast Asia to New Zealand and India.

Outbreak of Foot-and0-Mouth

Jia also defended China's handling of a foot-and-mouth outbreak on a farm northwest of Beijing that health and agriculture officials previously refused to confirm.

A total of 3,771 cattle were slaughtered this month to stop separate outbreaks in Beijing, two eastern cities and the northwestern region of Xinjiang, Jia said. That included 512 cattle on the farm in Yanqing County near Beijing.

Reporters who visited Yanqing County this week were detained briefly.

Governments are expected to report foot-and-mouth outbreaks promptly because other countries often block imports of animals from affected areas in order to protect their own herds.

The disease is not usually fatal, but herds often are slaughtered to prevent its spread. It affects cloven-footed animals such as cattle, sheep and pigs, causing blisters on the feet and mouth.

Jia said the delay in reporting was due partly to the time required to get laboratory confirmation. False reports could damage international trade, he noted.

"The Chinese government has no intention of hiding an outbreak of foot and mouth," he said.