China reported its first human cases of bird flu on Wednesday and said at least one person had died in a region where health officials have been battling the disease among poultry.

Confirmation of at least two human cases of avian influenza marked official recognition that the disease had been contracted here despite strenuous efforts in recent months to contain outbreaks in chicken, ducks and geese on farms across the country.

A World Health Organization spokesman, Roy Wadia, told reporters in Beijing that the human infections were not surprising but nevertheless represented a substantial threat in the world's most populous nation.

While bird flu is often lethal, it has not been highly transmittable from human to human. Health experts warn that the influenza virus could develop into a more contagious form and spark a pandemic. The disease has killed at least 64 people since 2003 in Vietnam, Thailand, Cambodia and Indonesia. The virus has also been carried by migratory birds to parts of Russia and Eastern Europe.

At least 42 people, about two-thirds of those killed by the disease, have died in Vietnam. The Vietnamese government, concerned that bird flu could spread widely during the winter, has announced that it will step up enforcement and monitoring in an effort to control the disease.

In China, the often close-quarters contact between farmers and the fowl they raise in their back yards and ponds has generated fears that, if an epidemic were to erupt, many villages could be ravaged by the disease.

To prevent a global outbreak, WHO officials said they would seek to contain the disease locally at the first sign it has acquired the ability to spread quickly among humans. Officials would have only days to quarantine the infected area and administer antiviral drugs. They stress that the swift detection of human infections is critical.

The Agriculture Ministry's chief veterinarian, Jia Youling, told reporters Tuesday that the government had decided to launch a vaccination campaign covering China's entire poultry stock, which amounts to billions of birds. Birds at some well-tended industrial poultry plants would not have to undergo vaccination, he said, but he described a mobilization campaign to reach the chickens and ducks often seen pecking around small farms across the countryside.

Millions of birds already have been vaccinated by Chinese authorities and thousands more have been destroyed since the illness emerged and threatened to spread into the human population.

The Agriculture Ministry, quoted by the New China News Agency, said a 9-year-old boy in the central province of Hunan tested positive for bird flu after falling ill with a fever and pneumonia-like symptoms. The boy has since recovered. His sister, 12, died on Oct. 17 from similar symptoms and was believed to have contract bird flu as well, the ministry said, but her body was cremated and officials were unable to scientifically confirm the cause of her death.

The boy previously had tested negative but was retested with help from WHO specialists dispatched to work with officials from the Agriculture Ministry, the lead agency in the campaign to contain the disease.

The second case confirmed by the ministry occurred in Anhui province in eastern China. A 24-year-old female farmer contracted the disease and died Nov. 10 after falling ill during an outbreak of bird flu in Anhui's Tianchang city that killed more than 500 chickens, it reported.

Ministry officials also reported that a woman in Liaoning province, in northeastern China, was undergoing tests in a hospital to determine if she was infected. She had contact with chickens believed to be infected.

Correspondent Alan Sipress in Jakarta, Indonesia, contributed to this report.

He Junyao, 9, tested positive for bird flu after falling ill. The boy has since recovered. His sister, 12, died from similar symptoms.