The flu has been moved to phase 6, which means the virus is being passed freely between people without easily traced chains of infection in two or more regions of the world.
The announcement warns the 194 member nations to prepare for the new flu strain which could infect almost one-third of the population in the first wave. WHO is also advising vaccine makers to develop the flu vaccine on a commercial scale as soon as possible.
The virus was first detected in April in Southern California when the Centers for Disease Control learned of two cases of a previously unknown strain. Mexico soon reported hundreds of infections and 68 deaths.
To date there are more than 13,000 confirmed cases in the U.S. resulting in 27 deaths.
The WHO has not yet recommended countries to limit travel or business. The flu appears to infect children and young adults although its true character hasn't emerged.
In the United States, 57% of confirmed cases are people between 5 to 24 years according to a CDC data.