BEIJING China's Health Ministry is puzzled by eight human cases of bird flu in January which appeared independent of any known case in birds, a spokesman said on Tuesday.
Five Chinese died from H5N1 in January in far-flung regions without any reported presence of the virus in birds on the mainland.
Dead birds that washed up in Hong Kong tested positive for the H5N1 strain this month, leading experts to question whether bird flu is widely present but undetected in China.
"We see the result, but not the cause. We don't know where it has come from, but people have been infected. When people are infected, in theory it should be present in birds," spokesman Mao Qunan told reporters.
Later on Tuesday, the Ministry of Agriculture said there had been an H5N1 outbreak among poultry in Hotan, in the far western region of Xinjiang, which had killed 519 birds.
Authorites have culled another 13,218 birds and the outbreak is under control, the official Xinhua news agency said.
China has reported one case of bird flu detected through sampling this winter, in eastern Jiangsu province. China conducts random sampling and culls birds when the virus is found.
Last week, the Agriculture Ministry defended its vaccination campaign as having successfully prevented widespread incidence of bird flu.
The Ministry of Health has also urged hospitals to increase efforts at early detection and testing for bird flu, Mao said.
Some people may be genetically more susceptible to bird flu than others, he added.
The mother of one of this year's victims, a toddler who has recovered, died of pneumonia shortly before her daughter took ill. But the woman was never tested for bird flu.
While H5N1 rarely infects people, experts fear it could mutate into a form people could easily pass to one another, sparking a pandemic.
(Additional reporting by Ben Blanchard)