The World Health Organization (WHO) on Saturday urged countries to be alert for unusual flu outbreaks after a deadly swine flu virus has claimed dozens of lives in Mexico and infected at least 11 people in the United States.

North America: dozens killed

On Saturday, Mexican Health Minister Jose Angel Cordova raised the probable death toll from the outbreak of the swine flu to 81, including 20 already confirmed, and said more than 1,324 had been suspected to be infected since April 13.

Mexico City has closed schools, museums and other public gathering places, and the Army has been distributing face masks to the population. The health minister said classes in the capital, the neighboring Mexico state and the northern state of San Luis Potosi will resume on May 6.

Mexican President Felipe Calderon has issued an emergency decree giving the government special powers to run tests on sick people and order them to be isolated.

In the U.S., two more cases of swine flu were confirmed on Saturday in the state of Kansas and one case in California, bringing the total number of people infected to 11. Eight more cases have been reported in Texas and California.

Eight schoolchildren in New York City were found to be infected with a type A influenza virus that was likely to be the swine flu.

Who issues alert

WHO says the virus from 12 of the Mexican patients is genetically the same as a new strain of swine flu, designated H1N1, which is also seen in eight people in California and Texas.

The H1N1, a mixture of swine, human and avian flu viruses, is still poorly understood by scientists.

Based on the advice of a WHO emergency committee, Director-General Margaret Chan "has determined that the current events constitute a public health emergency of international concern," the UN agency said in a statement on Saturday.

WHO urged all countries to boost their surveillance for any unusual outbreaks of influenza-like illness and severe pneumonia.

But the Geneva-based UN agency left the alert status at "Phase 3" denoting none or very limited human-to-human transmission on its scale of one to six. The alert status "Phase 4" would indicate evidence of an increase in human-to-human infection.

"It has pandemic potential because it is infecting people," Chan said in Geneva. "However, we cannot say on the basis of currently available laboratory, epidemiological and clinical evidence whether or not it will indeed cause a pandemic."