The official also urged stronger efforts to reduce the risk of transmission of the virus to new species of animals and to humans.

"What we really need globally, in the U.S. and abroad, is much stronger surveillance in animals, in wild birds, in poultry, in animals that are known to be susceptible to infection," WHO epidemiologist Maria Van Kerkhove told an online press conference.

The agency said it is in touch with partner agencies such as the World Organization for Animal Health and Food and Agriculture Organization to increase surveillance in animals.

The U.S. Department of Agriculture last month confirmed the presence of H5N1 bird flu in a pig on a backyard farm in Oregon.

Pigs represent a particular concern for the spread of bird flu because they can become co-infected with bird and human viruses, which could swap genes to form a new, more dangerous virus that can more easily infect humans.

"For us at the WHO we are always in a constant state of readiness as it relates to influenza, because it isn t a matter of if, it s a matter of when," Kerkhove said, adding that the risk to the general population for avian influenza remains low globally.

So far, 55 human cases of H5N1 bird flu, including in a child, have been reported in the United States this year, according to the U.S. Centers for Disease Control and Prevention.

Most of these cases were among farm workers who had contact with infected poultry or cows. There has been no person-to-person spread associated with any of the H5N1 bird flu cases, according to the CDC, but dairy and other farm workers are considered to be at higher risk of contracting the virus.