A pig at a backyard farm in the US state of Oregon tested positive for H5N1 avian flu, the first such case in the United States, officials said Wednesday.

The US Department of Agriculture said that the pig was found to be infected on Tuesday, four days after poultry on the same farm tested positive for the highly contagious virus.

"This farm is a non-commercial operation, and the animals were not intended for the commercial food supply," the agency said in a statement. "There is no concern about the safety of the nation's pork supply as a result of this finding."

Authorities euthanized the pig as well as four others kept on the farm "to facilitate additional diagnostic analysis," USDA officials said.

Two of the pigs tested negative for bird flu, while results are pending for the other two. The infected pig showed no signs of illness, according to the agency.

It said the infected livestock and poultry shared housing, equipment and water sources, which have enabled H5N1 transmission between species in the past.

The farm has been placed under quarantine to prevent further spread of the virus, and other animals on the farm, including sheep and goats, are under surveillance.

Public health experts have expressed concern about the growing number of mammals infected with avian flu, fearing the increased circulation could allow it to mutate into a form transmissible to humans.

However, genetic sampling from infected poultry on the Oregon farm "has not identified any changes to the H5N1 virus that would suggest... it is more transmissible to humans," the USDA said.

It added that it is conducting genomic sequencing of the pig's sample, with results pending.