BEIJING, May 21 (AP) -- Investigators have concluded that bird flu killed migratory birds in western China this month, but the virus did not spread to people or poultry, the government said Saturday.

The birds were found dead on May 4 in Qinghai province's Gangcha city, the official Xinhua News Agency reported, citing the ministry.

The ministry found the birds died of the deadly H5N1 flu strain, but "the disease did not spread to human kind or fowl," Xinhua said. It did not say how many birds had died, or where they came from.

A local official contacted by telephone said more than 100 migratory birds were found dead on an island in Qinghai Lake in early May. But the official of the Qinghai provincial animal husbandry bureau said he did not know whether those were the birds cited in the report.

The official, who would give only his surname, Shi, wasn't sure where the dead birds came from, but he said it might be southern China.

Health officials worry that some birds, which fly across China on migratory routes stretching from Siberia to New Zealand, could spread the virus to the country's vast population of ducks, geese and other domestic fowl.

In the past, China's government has temporarily banned visitors from nature reserves used by migratory birds in an effort to prevent the spread of the virus.

Some experts suggested migratory birds might have been the source of an outbreak of avian flu last year that prompted the slaughter of millions of fowl in 10 Asian countries.

But others say the few wild birds known to have gotten the virus probably contracted it from domestic poultry.