The World Health Organization is warning that more people in China are likely to become infected with the bird flu virus as winter sets in. Meanwhile, Indonesia reported the deaths of two women infected with the virus, which has killed more than 60 people since it reappeared in Asia two years ago.

Medical worker disinfects a poultry farm in Xuyi county, east China's Jiangsu Province

The warning of likely new infections came a day after China confirmed its first human cases of the potentially lethal H5N1 virus.

The World Health Organization's top official in China, Dr. Henk Bekedam, told reporters Thursday it is probably inevitable that new cases will turn up in the coming weeks as the weather gets colder.

"It's very simple," he said. "At the moment you have more poultry outbreaks, you will also expect more humans to be exposed to the sick poultry and therefore it can indeed happen."

Officials say China has a third of the world's poultry, and they say 70 percent of domesticated fowl are raised in backyard coops - unmonitored, and often living in close proximity to humans.

One of the confirmed Chinese victims is a young boy from Hunan province, who is still alive. His sister died, but no definitive tests were performed on her, and she is only listed as a suspected H5N1 victim. A woman, unrelated to the children, has also died of the virus.

While experts say conditions in China appear ripe for a major outbreak, and the despite the brother-sister connection, Dr. Bekedam emphasized Thursday there is no proof thus far of any human-to-human transmissions.

"One the one hand, it is very important that people need to be informed about some of the dangers. But you also have to be very careful that you don't go into a panic," added Dr. Bekedam. "One thing we have to realize is that the current virus is not easily transmittable to humans, and not transmittable between humans."

For now, most infections are known to have passed from infected animals to humans, and the rate of the spread has been low. However, scientists fear a worldwide pandemic if H5N1 mutates to a form that can easily spread between humans. The death rate for H5N1 is high, at about 50 percent - compared to half of a percent for more common flu strains.

Workers disinfect a road as villagers look on at a checkpoint at Qitaizi Village, in Heishan, in China's northeast Liaoning province

Chinese officials are rushing to cull hundreds of millions of chickens and ducks in about a dozen areas where there have been confirmed bird flu outbreaks. They have also announced plans to vaccinate more than five billion chickens, ducks and geese against the virus.

A laboratory in Hong Kong on Thursday confirmed the flu killed two more people in Indonesia recently, bringing the number of bird flu deaths in that country to seven.