New York (PTI): The situation in Indonesia -- the country worst hit by bird flu -- remains "grave" despite major efforts to control the outbreak of the disease, the United Nations Food and Agriculture Organisation (FAO) has warned.

Avian influenza has become "deeply entrenched" in Indonesia with 31 out of 33 provinces being infected, according to FAO. The virus is endemic in Java, Sumatra, Bali and southern Sulawesi with sporadic outbreaks reported from other areas.

Just last month, five people living in the west of Java, Indonesia's most populous island, died after contracting the H5N1 virus.

"The human mortality rate from bird flu in Indonesia is the highest in the world and there will be more human cases if we do not focus more on containing the disease at source in animals," said FAO Chief Veterinary Officer Joseph Domenech.

Domenech warned that the high level of virus circulation in birds in the country could create conditions for the virus to mutate and to finally cause a human influenza pandemic.

"The avian influenza situation in Indonesia is grave - all international partners and national authorities need to step up their efforts for halting the spread of the disease in animals and making the fight against the virus a top priority," he stated.

Among the major constraints the country is facing in tackling the virus are a highly decentralised administration, lack of engagement with commercial poultry producers and lack of resources for veterinary services and control campaigns.