The Chinese government says tests have found that its pigs are clear of a highly virulent strain of bird flu.
Scientists fear more humans could be infected in the future
The statement comes several days after a Chinese scientist said cases of the H5N1 strain of the virus had been discovered in pigs in 2003 and 2004.
The World Health Organization said it was seeking clarification on the issue.
If the spread of bird flu to pigs is confirmed, there is a fear that the virus could mutate into a strain more easily transferable to humans.
Almost 200 million birds were culled during a flu epidemic in Asia earlier this year, and more than 20 people died in some of the relatively rare cases when the virus spread from the birds to humans.
But WHO experts warn that, if there are pigs harbouring both bird and human flu viruses, the two strains could interact to create a more deadly strain.
An official at the China National Avian Flu Reference Laboratory said in a conference on Friday that the H5N1 virus strain had been found in pigs at several farms in the country.
Chen Hualan 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.
But 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.
It did not, however, directly refute Ms Chen's comments about the case last year.
It also did not make clear whether the tests were carried out on laboratory animals or on farms.
Just as the Chinese Agriculture Ministry was releasing its statement, a senior WHO official told a news conference in Malaysia that finding H5N1 in pigs would increase the chances of a flu pandemic.
The WHO said it was seeking clarification of the situation, and urged the Chinese Agriculture Ministry to discuss the matter in an open manner.
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 in Kuala Lumpur announced earlier this week that the H5N1 strain had been found, prompting Singapore to ban all poultry imports from Malaysia with immediate effect.