China will provide four samples of human bird flu virus to the WHO - the second such contribution from the country, according to Wang Yu, director of the Chinese Center for Disease Control and Prevention.

"The handover is still in process," Wang said on the sidelines of a health conference in Beijing.

China forwarded two bird flu virus samples along with other virus information to the WHO in December.

Wang said laboratory tests of China's human victims show there have been no mutations involving the deadly H5N1 strain of avian flu.

But, he stressed, the two most recent cases in Guangzhou and Shanghai have posed a new challenge for China, because the previous victims were in rural parts of the country and their illnesses were believed to have been caused by poor health conditions and lack of proper hospital facilities.

"Although there's no evidence that the two patients were exposed to dead poultry, it doesn't mean there's no virus in the poultry in the city environment," said Zhang Wenqing, who works in the WHO's global influenza program.

China has reported 16 human cases of bird flu, including 11 deaths.