It is the highest level since the U.N. agency initiated its six-point scale in 2005 in response to the avian influenza crisis. The increase in the alert status signals an outbreak of human-to-human transmission in at least one country, which increases the risk of a global epidemic, but does not mean a pandemic is inevitable, WHO officials said in a news release.
"Given the rapidly evolving situation (it was felt) that it was important to give a strong signal to countries that now is a good time to strengthen preparations for possible pandemic influenza," WHO Assistant Director-General Keiji Fukuda told reporters in Geneva, Switzerland.
Fukuda said an emergency committee of health experts raised the alert level because the virus had already spread to the United States, Mexico and Canada, with a verified case in Spain.
Fukuda said authorities need to focus their efforts on mitigating the disease rather than attempting to stop its spread, stressing that "containment is not a feasible consideration." WHO would not recommend closing borders or restricting travel, he said.
Worldwide, the number of confirmed cases was 73, WHO officials said. A WHO representative said the breakdown by country was 40 in the United States, 26 in Mexico, six in Canada and one in Spain, CNN reported.
In Mexico, officials Monday said the infection was suspected of causing as many as 149 deaths from nearly 2,000 cases, The Washington Post reported.
In the United States, officials Monday confirmed 20 new cases, all at a New York school in which eight U.S. cases were confirmed earlier. Suspected cases also were reported in Brazil, New Zealand, France, Israe, Scotland and South Korea
U.S. President Barack Obama said the swine flu outbreak is a "cause for concern" requiring a heightened state of readiness, but not a "cause for alarm."
Still, federal health officials recommended Americans delay unnecessary travel to Mexico, the Post said.
The European Union's health commissioner called on people to avoid traveling to the United States and Mexico.
Swine flu is a contagious respiratory disease affecting pigs and is caused by a type-A influenza virus, CNN said. The strain is a new variation of an H1N1 virus, a mix of human and animal versions.
When the communicable flu spreads among humans, instead of from animals to humans, it can mutate, making it harder to treat or fight off because people have no natural immunity, health officials said.
The United States Sunday declared a "public health emergency," and civilian and military stockpiles of antiviral drugs were being readied for rapid distribution if necessary.