China denied it encouraged farmers to use a drug meant for humans to stop the spread of bird flu in poultry which could render the medicine useless, but said it would investigate, the AFP news agency reported.

A ministry of agriculture official refuted a Washington Post report claiming Chinese farmers, with government encouragement, have widely used amantadine to combat deadly bird flu in poultry since the 1990s.

"The government has never permitted this drug to be used on poultry," Xu Shixin, director of the veterinary bureau, told AFP.

He refused to say whether Chinese farmers, motivated by a desire to reduce losses, were nonetheless using the antiviral drug to treat infected poultry instead of culling them, the recommended practice.

"We don't know about this situation," Xu said.

However in an interview with the official China Daily, Xu seemed to acknowledge the antiviral drug could have been used by some farmers on their flocks and said the ministry planned to dispatch inspection teams nationwide to stop the drug from being used on poultry.

"We'll take measures soon to curb the action," he was quoted as saying, without elaborating.

The government would also supply farmers with vaccines, which are cheaper and more effective, to replace the use of amantadine by some farmers, the paper cited Xu as saying.

The UN Food and Agriculture Organization and the World Health Organization both said they were seeking clarification from China.

Amantadine is one of only a handful of medications available for treating human influenza, and is approved only for use in humans, experts said.

The Post cited international researchers saying the use of amantadine on poultry had caused it to lose its effectiveness to protect humans and meant it would no longer protect people in the event of a worldwide bird flu epidemic.

So far bird flu is mainly transmitted between birds but it is feared the virus can mutate to a more lethal form that can be easily transmitted to humans.

The H5N1 bird flu strain circulating in Vietnam and Thailand, the two countries hardest hit by the virus, already had become resistant to amantadine.