'What we can say in general is that any misuse of drugs, and especially antivirals which are key in the fight against pandemics ... would be very disappointing indeed. Drugs intended for one use should be very carefully monitored in their use.'

The Washington Post said Chinese farmers used amantadine widely in the late 90s, putting it in their flocks' water supply to suppress bird flu outbreaks. Researchers quoted suggested this led to the rise of a resistant form of the deadly H5N1 flu strain.

"We've raised this with the Ministry of Health," said Roy Wadia, a WHO spokesman in Beijing. "They said they were fully aware of the issue and would respond to us."

Worry That Bird Flu Might Mutate

The UN Food and Agriculture Organization asked China's Agriculture Ministry about the report, Wadia said.

The health and agriculture ministries didn't respond to requests Monday by The Associated Press for comment.

"This is a matter of concern if these studies are proven to be correct and this drug is not useful in the fight against H5N1 in humans," said Wadia.

Health experts worry that bird flu might mutate into a form that can spread directly from person to person, setting off a pandemic that could claim millions of lives. People killed by the flu so far have contracted it from sick birds.

Drug-resistant forms of H5N1 have been found in Thailand and Vietnam, according to researchers.

"What we can say in general is that any misuse of drugs, and especially antivirals which are key in the fight against pandemics ... would be very disappointing indeed," said Wadia. "Drugs intended for one use should be very carefully monitored in their use."

China Has No Cases in Humans

The rise of a flu strain resistant to amantadine has forced doctors to use an alternative drug that is more expensive and harder to manufacture, the Post reported.

Asia's latest bird flu outbreak began late last year and has killed 38 people in Vietnam, 12 in Thailand and four in Cambodia.

China has no cases in humans but two outbreaks in the country's remote west killed more than 1,000 migratory wild birds last month.

WHO experts were traveling to the western province of Qinghai this week with Chinese government experts to inspect the site of one outbreak.