Scientists in China say they have discovered a highly virulent strain of bird flu virus in pigs.
Thousands of birds have already been infected with avian flu
An official at the China National Avian Flu Reference Laboratory said the H5N1 virus strain had been found in pigs at several farms in the country.
More than 20 people died and almost 200 million birds were culled during a flu epidemic in Asia earlier this year.
The spread to pigs has yet to be confirmed, but there could be serious implications for human health if it is.
The World Health Organization said that if the pigs were harbouring both bird and human flu viruses, the two strains could interact to create a strain capable of transferring easily to humans.
Chinese scientist Chen Hualan first announced the existence of bird flu in pigs during a conference speech on Friday.
She later told journalists that the virus had been discovered in pigs in south-east China's Fujian province in 2003, and in "another place" in 2004.
Officials at both the WHO and the Food and Agriculture Organization (FAO) said they were unaware of the new development until Ms Chen's comments.
"I think it's something we've long warned can happen. I don't think we're shocked, but we need more details," WHO spokesman Roy Wadia told the French news agency AFP.
The deadly H5N1 strain of bird flu is currently capable of spreading from poultry to people, but the incidence of cross-species transfer is relatively rare and so far there have been no cases of human-to-human transfer.
Scientists fear more humans could be infected in the future
There is a fear that, if it has spread to pigs, the virus could mutate and form a strain that could then readily transfer to humans.
But the WHO cautions that a lot more information is needed before an accurate risk assessment can be made.
For one thing, it is not yet clear whether the pigs are actually infected with the virus, or have merely come into contact with it.
Pigs can pick up viruses in their snouts from sniffing at the ground, but this does not mean they actually have the disease.
In an earlier outbreak, nasal swabs from pigs in Vietnam tested positive for bird flu, but blood tests proved that they were not actually infected.
"If we found [the virus] in the nostril, a superficial part of the body, it would not be as significant. If we found it in an organ, say the lungs, that would be significant," FAO spokesman Juan Lubroth told AFP.
Meanwhile bird flu continues to plague many parts of Asia.
It ravaged poultry flocks throughout the region earlier this year, and caused the deaths of 27 people in Vietnam and Thailand.
A further three people are said to have died from the disease in Vietnam earlier this month.
Malaysia is the latest country to report the incidence of the disease in its poultry flock.
Officials announced earlier this week that the H5N1 strain had been found, prompting Singapore to ban all poultry imports from Malaysia with immediate effect.