China is investigating whether three people, including one who died, were the country's first known human cases of the bird flu.

According to the official China Daily newspaper, China's health ministry says 192 people who had contact with the three potential victims have been put under medical observation.

One of those who had contact with the three has developed acute bronchitis.

China's official news agency said that all three have tested negative for the virulent H5N1 strain of bird flu, but experts said the disease couldn't be ruled out.

A spokeswoman for the World Health Organization said that the WHO will be working with Chinese experts to further analyze the samples.

"What China has done is to continue that investigation, to continue to take tests and now they are reporting that on the 9-year-old boy, there's something that is called a 'suspicious positive.' That's not a confirmation at this stage, but it does require further investigation and they've requested assistance from the WHO with this ongoing investigation," said Dr. Julie Hall, WHO spokeswoman.

Meanwhile, Chinese officials have ordered all live poultry markets in Beijing to close immediately.

Market stalls selling poultry meat in Beijing's markets will also face tougher inspections by health officials. Markets are allowed to sell only meat that has been processed in approved facilities.

The increase in anti-bird flu measures in China came as a meeting of hundreds of international experts in Geneva opened with warnings that a global human flu pandemic was inevitable.