The WHO said Tuesday that an increasing number of humans are at risk of catching the bird flu as the disease continues its rapid spread in poultry flocks across Asia.

A few dozen people in Thailand and Vietnam have contracted the disease - but many neighboring countries have reported outbreaks in chickens and are therefore at risk of human infections. About 20 people have died of the virus so far this year.

The United Nations health agency is particularly worried about China. Although no human cases have yet been found there, an increasing number of provinces have confirmed outbreaks among poultry.

"This infection is so widespread across great swaths of the country," said WHO Mainila spokesman Peter Cordingley, referring to China. "The Chinese authorities tell us they have no reported cases of infection in humans. We think it is quite conceivable that there are human cases that have not been detected."

Mr. Cordingley says part of the problem in China is that health surveillance systems are weak in poor, rural provinces and human cases might go unnoticed.

In at least 10 Asian countries, millions of chickens have been slaughtered and buried in the effort to stamp out bird flu viruses. One type of virus also has been found in the United States.

Scientists say this is probably the worst flu outbreak in birds on record, but the disease does not spread easily between people. There is, however, a risk that this flu, which is caused by the H5N1 virus, could pick up human flu traits and cause a global pandemic in people.

United Nations scientists are keeping a close watch on three hospitals in Vietnam for new cases or a change in the way the disease spreads. A U.N. official in Vietnam said Tuesday the disease is far from being contained in rural areas.