DONALD G. McNEIL Jr., The New York Times, reports:

Two reported new outbreaks of avian flu among birds in western China have raised fears that the virus is being spread widely by migrating birds and mutating rapidly.

The regional director for the World Health Organization, Dr. Shigeru Omi, told reporters in Beijing yesterday that the two recent outbreaks in remote areas in which hundreds of birds died were worrisome because they involved migratory waterfowl and domestic geese, birds that until now had been fairly resistant to the disease.

More than 13,000 geese were destroyed in Tacheng, in the Xinjiang autonomous region, after about 500 died of H5N1 avian flu, China’s Agriculture Ministry reported.

Poultry markets were closed and roadblocks set up in the area, the official Xinhua news agency said.

In late May, the government reported that hundreds of bar-headed geese, gulls, ducks and cormorants had been found dead on an island in a salt lake in the Qinghai region that lies on an important migratory route.

Previously, the H5N1 flu had been lethal to domestic chicken flocks, but veterinary officials had believed that geese and wild birds carried the disease without dying of it.

“The best thing I can say is to keep our vigilance high,” Reuters quoted Dr. Omi as saying.

For the last two weeks, rumors circulated on some Web sites tracking infectious diseases that more than 120 people, including six tourists, had died of avian flu in Qinghai, and that hundreds had been quarantined.

However, they all proved traceable to a site run by antigovernment dissidents, which said it could not verify information members had posted anonymously. Pictures on the site purporting to show hundreds of dead birds were grainy, and allegations that the site’s “reporters” had been arrested were unconfirmed.

“We’re now more skeptical of the sourcing than we were,” said Bruce Klinger, an analyst for the Eurasia Group, a consulting firm that drew attention to the reports and then contacted American diplomats in China in an effort to confirm them.

A government spokesman said there had been no human deaths there, and The Associated Press reported that the health minister had given the W.H.O. officials permission to visit the sites of the reported bird deaths.

As of Wednesday, according to the W.H.O., there were 54 known deaths from avian flu in the world, all in Cambodia, Vietnam and Thailand.