(CNN) -- The United States government declared a public health emergency Sunday as the number of identified cases of swine flu in the nation rose to 20.
Additional cases of swine flu are expected to be reported in the coming days, added Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention.
The World Health Organization advised all countries to be on the lookout for "unusual" outbreaks of flu, after an emergency meeting Saturday as the seriousness of the outbreak became clear.
By Sunday, 81 deaths in Mexico had been deemed "likely linked" to swine flu. Viral testing has confirmed 20 cases, said Dr. Jose A. Cordova Villalobos, Mexico's health secretary.