Dr. Margaret Chan, the U.N. agency's director-general, said the decision to raise the alert on the 6-point scale indicated that all countries should "immediately" activate pandemic preparedness plans.
"This change to a higher phase of alert is a signal to governments, to ministries of health and other ministries, to the pharm industry and the business community that certain actions now should be taken with increased urgency and at an accelerated pace," Chan said.
The announcement came as the number of confirmed cases increased rapidly across the world.
The WHO and national governments have confirmed 148 cases in nine countries. Most of those cases are in the United States, where the Centers for Disease Control and Prevention has confirmed 91 cases.
The figures include seven deaths in Mexico and one in the United States. More than 2,700 other patients worldwide are believed to be suffering from the virus that government officials call by its technical name, 2009 H1N1.
The first cases of the virus were detected in Mexico, where health officials suspect swine flu in more than 150 other deaths and roughly 2,500 illnesses. As of late Wednesday, 99 cases had been confirmed -- up from 26, Mexico's health secretary reported. An eighth fatality was also confirmed. iReport.com: "Regular life" in Mexico with masks
The additional cases and fatality were not immediately confirmed by WHO
