The World Health Organisation said it was sending experts to southern China to help investigate whether bird flu killed a 12-year-old girl last month.
Chinese authorities quarantined 116 people after the latest outbreaks on Sunday of the deadly H5N1 strain of the virus killed 1100 chickens in Fuxin and Jinzhou, cities in northeastern Liaoning province, the Agriculture Ministry said on Thursday.
A case there two weeks ago prompted officials to destroy more than 6 million birds.
China earlier warned that counterfeit vaccines were being sold in Liaoning, raising the possibility that millions of chickens, ducks and other birds, which farmers think are inoculated, might still be susceptible.
China suffers from rampant counterfeiting of food and medicines.
"Quite clearly, there's a major problem in Liaoning, and it seems from what the Chinese are saying this has to do with using shoddy, inferior or maybe fake vaccines for poultry," said a WHO spokesman, Peter Cordingley.
China has reported six bird flu outbreaks in the past month
"And what we have now, almost certainly, we think, is sick chickens who are showing no symptoms, and that is very, very bad. They are silent carriers of the virus," he said.
The latest outbreaks raised to six the number reported in China in the past month. It usually takes the government several days to confirm the cause of death in birds.
Authorities in Liaoning destroyed 670,000 birds after the outbreak on farms near Fuxin and Jinzhou, the official Xinhua News Agency reported.
China has also reported outbreaks in poultry in the Inner Mongolia region in the north, and in the provinces of Anhui in the east and Hunan in central China.
No human case has been reported, but experts say it's inevitable if China can't stop outbreaks in poultry.
In Kuwait, a senior official said the country had found two cases of bird flu but was still testing whether they were the deadly H5N1 strain or a weaker variation.
The first case was an imported bird found at the Kuwait City airport, while the second was a migrating wild fowl found on a beach, said Sheik Fahd Salem Al-Ali Al Sabah, the chairman of Kuwait's Public Authority for Agriculture and Fisheries.
Mohammed al-Mihana of the Public Authority said tests indicated the strain was H5, but it was not determined whether it was N1 or N2. The H5N2 strain is believed to cause little illness.
The virus has killed at least 63 people in Asia. Hardest hit has been Vietnam, where bird flu outbreaks have been reported in at least six provinces in Vietnam in the past month, killing or prompting the destruction of more than 130,000 birds.
H5N1 first appeared in Hong Kong in 1997 but was curbed when authorities destroyed all poultry in the territory. It re-emerged in December 2003, and has recently spread from Asia to Europe.