China has confirmed reports that the deadly H5N1 strain of bird flu has been found in its pig population.
Scientists fear more humans could be infected in the future
Agriculture ministry officials said two infected pigs had been discovered, one in 2002 and one in 2003.
But the cases are said to be isolated, and surveillance measures are being stepped up in the relevant areas.
The WHO fears that if pigs can harbour both bird and human flu viruses, the two strains could interact and form a strain that more easily infects humans.
A bird flu epidemic has hit Asia this year, causing the deaths of more than 20 people and forcing farmers to cull almost 200 million birds.
The H5N1 strain is only rarely spread from birds to humans, and there have so far been no cases of it being transmitted from human to human.
But there is a fear that an epidemic of a new mutated form of the virus could be much more dangerous.
The furore began last Friday, when a Chinese scientist first admitted that cases of the H5N1 strain of bird flu had been found in pigs.
The announcement took the World Health Organization by surprise, and it said it was seeking clarification on the issue.
In an internet statement on Monday, the Chinese Ministry of Agriculture said it had tested 1.1 million poultry and pork samples for H5N1 earlier this year, and found nothing in pigs.
Millions of chickens were culled earlier this year
But on Thursday, the government confirmed that pigs had been infected with bird flu in the past.
Jia Youling, director general of the Ministry of Agriculture's veterinary bureau, told a Beijing news conference that the two infected pigs had been found in the same area of China. But he declined to say where, citing the need to protect the interests of the local population.
Mr Jia played down the two cases, saying no pigs had actually fallen ill from bird flu, and there was no evidence of the virus being transmitted to humans.
"The H5N1 virus infecting pigs is a chance, individual occurrence. There is no evidence yet the H5N1 bird flu virus can be passed from pig to pig, or of pigs infecting people," Mr Jia told Reuters news agency.
"Should an epidemic occur, we will follow the guidelines of the relevant international organisations and report it in a timely fashion," he said.
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 in Kuala Lumpur announced earlier last week that the H5N1 strain had been found, prompting Singapore to ban all poultry imports from Malaysia with immediate effect.