a is asking for foreign assistance to help it confront the world's first major outbreak of avian flu in wild birds, an outbreak that threatens to spread thousands of kilometres next month when the birds begin to migrate.
Every day, an average of 20 wild birds in China's remote Qinghai Lake drop dead from a mystery strain of bird flu. And every day, health experts grow more worried about the 100,000 migratory birds from the same lake that could carry the virus to new places and ultimately to humans.
In a race against time, the experts are hoping to test or monitor thousands of the birds before they leave on their annual migration in late August and early September. So far, only a dozen birds have been tested. Of the 189 species of wild birds at the infected lake, 184 have never been tested.
Many of the untested birds could be quiet carriers of the virus, spreading it to other wild birds and domestic animals without showing any symptoms.
"We are saying very strongly that the unaffected birds need to be tested," said Julie Hall, a disease-control specialist at the World Health Organization office in Beijing.
The outbreak began in early May at a bird sanctuary at Qinghai Lake, a famed saltwater lake in the province of Qinghai, in northwestern China.
Yesterday, the WHO revealed that the outbreak has already killed 5,000 geese, gulls and other wild birds at the sanctuary, five times as many as the Chinese authorities had previously acknowledged.
"This is the first time we've seen large numbers of migratory birds dying from bird flu," Dr. Hall said. "The virus has obviously changed to be more pathogenic to animals. What it means to humans we don't know."
Early tests show that the birds are dying of the same H5N1 virus that killed 54 people in southeastern Asia and forced the slaughter of millions of chickens. The exact gene sequence of the virus has not been released, even though a Chinese laboratory has managed to isolate and identify it.
The WHO says it wants to see the sequence information as soon as possible, in order to investigate whether the virus spread to China from neighbouring Vietnam, Thailand or some other source.
It is "vitally important" for the sequence to be shared and China has promised to do so, Dr. Hall said.
Although most of the world's bird-flu deaths have occurred in Vietnam so far, the WHO is worried about China because millions of Chinese farmers live in close proximity to billions of chickens and ducks.
About 70 per cent of China's poultry are raised in "backyard" operations, the WHO estimates.
Health experts are troubled by signs that the disease seems to be "evolving," especially in Vietnam. If the virus mutates into a form that is easily transmitted in humans, it could kill millions of people worldwide.
"We remain concerned about the threat of a pandemic," Dr. Hall told a news conference yesterday. "The virus is unstable, unpredictable and very versatile. It's very difficult to predict when a pandemic might happen."
Another possible outbreak of bird flu was reported this month among 2,177 geese in Xinjiang, another northwestern province of China. So far the Chinese government has not allowed the WHO to visit the site of the outbreak.
"We are very eager to go to Xinjiang," said Henk Bekedam, the chief WHO representative in China.
Since the start of the outbreak in Qinghai, the bird sanctuary has been sealed off and guarded with a series of quarantine cordons that extend as far as 50 kilometres around it. The tightest cordon is within 10 kilometres of the lake. Anybody venturing within 30 kilometres of the lake must wear protective equipment and be sprayed with disinfectant when leaving the cordon.
Authorities have culled 20,000 domestic poultry within a 20-kilometre radius of the lake. But the wild birds have not been culled, partly because some are rare species.
China has a narrow "window of opportunity" to track and tag the potentially infected birds before they disappear on their migratory routes, the WHO has warned.