By Matt McGrath
Science reporter, BBC News, Rome
China, Indonesia and African nations are under-reporting incidences of bird flu, according to the World Organization for Animal Health (OIE).
Under-reporting may be due to inadequate compensation funds
A lack of adequate compensation schemes for farmers with infected poultry is the major factor, said the OIE avian influenza coordinator.
She is urging developed countries to provide the funding for such schemes.
The call came at the end of a two-day international conference to discuss the spread of avian flu.
Speaking at a news conference, Dr Christianne Bruschke said that under-reporting was happening for a variety of reasons in different parts of the world.
In Africa, there are problems of time, distance and education.
"Farmers will probably not report sick animals. We also think that people in Africa may not recognise the signs of the disease.
"Their veterinary services are very weak and many countries do not have laboratory facilities - we have all the ingredients there that could lead to under-reporting."
There are also major problems with Indonesia, where human deaths from bird flu are a reflection of serious problems with animals.
The disease now appeared to be permanently infecting poultry in the country, said Dr Bruschke, and this made accurate reporting of cases all the more difficult.
"I think it could be the case because in certain regions the virus is getting more or less endemic; so in regions like Java, they might not report every single outbreak anymore," she explained.
Dr Bruschke stressed that almost all countries were willing to report and acknowledge how serious a crisis this was.
"China is openly communicating with us and cooperating - but it is a very big country.
"We sometimes see the outbreaks in wild animals - they will not always detect them. There is also not a very good compensation scheme in place so we feel there might be under-reporting. "
The OIE is encouraging developed countries to provide funding for compensation schemes. There have been discussions at recent meetings of donor countries in Geneva and Beijing but no concrete proposals have emerged as yet.
More than 300 scientists from 100 countries were meeting in Rome to discuss the impact of wild birds on the spread of avian influenza.