The Food and Agriculture Organization (FAO) warned on Wednesday that killing wild birds is not an appropriate measure to control the spread of bird flu, which has ravaged domestic poultry farming in nine Asian countries and claimed 20 human lives.

What is needed is a prevention system based on control and surveillance to ensure that any contact between wild birds and poultry is avoided or at least monitored, the Rome-based agency said in a press release distributed in United Nations.

"Experience has shown that this has been a good strategy and that the destruction of wild birds is unnecessary," FAO said.

The outbreak of the H5N1 strain of bird flu has already led to the culling some 80 million chickens but the agency reported last week that the spread of the virus was still not under control.

Among measures FAO recommended for preventing infection from wild birds are steps by commercial poultry owners to ensure that poultry pens and drinking water cannot be contaminated by migrating species. If this cannot be done, then measures should be taken to ensure that the drinking water is safe.

Such steps include erecting pens to keep domesticated poultry away from wild birds and keeping domestic waterfowl separate from poultry where both have access to the same water sources.

Commercial poultry producers should also apply measures such as maintaining a high level of security regarding all traffic coming onto poultry farms and a very high standard of hygiene to minimize the disease's spread.

"Good biosecurity is a must," FAO said. "If we understand when, how and where wild birds migrate, then we are better prepared and know when we should be more vigilant and have better surveillance in place."

The disease has spread to humans in Vietnam, which has reported 21 confirmed cases, 14 of them fatal, and Thailand with 8 cases, 6 of them fatal.

Countries where outbreaks of the H5N1 strain in birds have so far been confirmed are Cambodia, China, Indonesia, Japan, Laos, Pakistan, South Korea, Thailand and Vietnam.