Global environmental group Birdlife International said wild birds have been unfairly blamed for the virus.

It says human practices like the trade in poultry and wild birds, and modern agricultural methods, probably play a major role in spreading the virus.

The FAO, which is monitoring the global spread of bird flu, supports the practice whereby faeces from farm animals are used to boost fish production.

The excrement is used to boost nutrients in water for the organisms the fish feed on.

Domenech told Reuters there was a theoretical risk of fish farms becoming a source of infection if excrement from infected poultry were poured into the ponds.

It could create "an infection outbreak in the environment, in the water, which can be the source of contamination of other birds which come to drink there".

He added, however, that as long as the correct surveillance was in place, infection should not happen, or could be dealt with quickly if it did.

"To ban these systems of raising livestock which are extremely efficient and irreplaceable to feed the populations in those countries, would be like banning the raising of ducks because ducks are considered one of the main sources."

The FAO, which has warned Europe could be at risk of bird flu when migratory bird return from the south in spring, is tracking outbreaks of the flu to determine how it is spread.