A laboratory in China says it has discovered a deadly strain of the bird flu in pigs, raising concern among scientists that the virus may be closer to changing into a form that is more easily transmittable to humans.

World Health Organization, WHO, officials were quick to caution against any panic over the revelations Friday by Chen Hualan, a bird flu researcher at China's Harbin Institute of Veterinary Medicine.

Ms. Chen told participants at a Beijing conference on bird flu that the H5N1 virus has been found on pig farms in China over the past two years. Her comments raise concerns that the virus, which has struck mainly chickens and other poultry, may have changed enough to easily make the jump to mammals.

World Health Organization officials, however, say not enough information is available to prompt worry that humans may soon be easily infected. Roy Wadia with the WHO office in Beijing says, while pigs can serve as sort of mixing vessels, in which viruses can become passable to humans, there is no indication that all of the elements for this to happen with H5N1 are present.

"A pig must be infected by both human and bird flu strains at the same time for this to actually happen, and that can occur just by a chance," he noted. "Such a chance is really small, unless a strain is widely circulating in pigs, in other words, pig-to-pig transmission, and we don't know if that's happened yet."

Chinese officials did not say where the pigs were infected or how many had been.

H5N1 is the first avian flu virus known to jump directly from birds to people. Most flu viruses are thought to move from birds to pigs or other mammals before spreading to humans. The first outbreak occurred in 1997 in Hong Kong, and six people died.

An outbreak of the H5N1 bird flu devastated the poultry industry in a number of Asian countries earlier this year, but its spread among humans was limited. Twenty-seven people have died in Thailand and Vietnam after catching the virus. The victims reportedly had all been in contact with infected birds.

Malaysia this week went on a nationwide alert after the virus was found in poultry near the border with Thailand. Officials in Thailand on Friday said the virus had been discovered in ducks, prompting them to cull thousands in an effort to stop the outbreak.

WHO officials on Friday said that, while there is no information now to suggest the disease can easily spread to humans, it is important for nations to be prepared to deal with a possible outbreak.