Globally, countries are being asked to increase reporting and surveillance of the disease, which is implicated in human deaths in Mexico and at minimum 20 non-fatal cases in the US.
"It would be prudent for health officials within countries to be alert to outbreaks of influenza-like illness or pneumonia, especially if these occur in months outside the usual peak influenza season," WHO director-general Margaret Chan said.
More than 100 people have died from pneumonia caused by a flu-like illness in Mexico, WHO said, and more than 1,600 people have symptoms of the illness. Some deaths have been confirmed as the flu, which combines bird, pig and human strains.
US officials report 20 people were infected with swine flu in California, Texas, New York, Kansas and Ohio, but none of the cases have been fatal.
President Obama said the outbreak is a "cause for concern and requires a heightened state of alert," but is not a "cause for alarm," according to a story from CNN.
He added the federal government is monitoring the situation and declared a public health emergency as a "precautionary tool to ensure that we have the resources we need at our disposal to respond quickly and effectively."