|Cages containing chickens at a live bird market in Beijing (File photo)|
Chinese officials had no immediate comment on the report, which first appeared in the U.S. newspaper The Washington Post.
The newspaper said that Chinese farmers, with the encouragement of the government, violated international livestock guidelines by feeding the drug Amantadine - intended for humans - to chickens during the past several years.
That has experts concerned the drug would no longer help humans fight off the disease in the event of a worldwide avian-influenza epidemic.
The World Health Organization was among a number of agencies asking Chinese authorities for an explanation.
The WHO representative in China, Dr. Henk Bekedam, says it is too early to tell what, if any, action might be taken against the Chinese government.
"I am not sure if it is violating something very clear," he said. "But, indeed, the concern is that, if we use anti-virals - drugs that could be used for humans - if they are used not properly, while you deal with the same virus in animals, by the time they [are] used for humans, they might not be working."
International health officials began suspecting that human anti-viral drugs were being administered to chickens last year, when researchers discovered that the bird flu virus, known as H5N1, had become resistant to Amantadine.
The drug is one of two in the world that are used to treat influenza. The other is much more costly and difficult to manufacture, and officials are worried it may be out of reach for many people in the event of a major outbreak.
Avian influenza has killed at least 54 people in Cambodia, Thailand, and Vietnam. Almost all the victims caught the disease directly from birds. But the World Health Organization is concerned that the virus might undergo mutations that would allow it to spread more easily among humans.