China faces a "grim" situation in preventing and controlling human cases of bird flu, the health minister said, after announcing four human infections in the last two weeks and three deaths.

Health Minister Chen Zhu called for hospitals to spare more resources in diagnosing and treating bird flu and more cooperation between agriculture authorities and his ministry, Xinhua news agency said.

A Chinese newspaper reported that the mother of a toddler diagnosed with bird flu had died of severe pneumonia earlier this month, but no samples had been taken to see if she had bird flu. She had been in contact with poultry before her death.

The toddler, and the three recent fatalities, have all fallen ill in areas where there have been no known cases of bird flu in birds. China vaccinates heavily for bird flu, raising concerns among some experts that the vaccines could be masking the presence of the virus.

"The current cases are separate cases. There's no connection," Shu Yuelong, vice director of virus control and prevention with the National Centre for Disease Control and Prevention, was quoted by Xinhua as saying.

"But these cases warn us to improve prevention and supervision over the epidemic and ensure early detection and diagnosis when new cases are found."

The H5N1 strain of flu remains largely a virus among birds, but experts fear it could change into a form that is easily transmitted among humans and could spark a pandemic that could kill millions worldwide.

Since the H5N1 virus resurfaced in Asia in 2003, it has infected 391 people, killing 247 of them, according to WHO figures released in mid-December.

At least 34 people have been infected in China and 23 have died.