The suspected number of deaths rose to 149 in Mexico, the epicenter of the outbreak with nearly 2,000 people believed to be infected. In the United States the number of confirmed cases rose to at least 50. Worldwide there were 73 cases, including six in Canada, one in Spain, and two in Scotland.
While the total cases were still measured in hundreds, not thousands, Mexican Health Secretary Jose Angel Cordova said the epidemic was entering an extremely dangerous phase, with the number of people infected mushrooming even as authorities desperately ramped up defenses.
The WHO raised the alert level to Phase 4, meaning there is sustained human-to-human transmission of the virus causing outbreaks in at least one country. Its alert system was revised after bird flu in Asia began to spread in 2004, and Monday was the first time it was raised above Phase 3.
"At this time, containment is not a feasible option," as the virus has already spread to several other countries, said WHO Assistant Director-General Keiji Fukuda.
Putting an alert at Phases 4 or 5 signals that the virus is becoming increasingly adept at spreading among humans. That move could lead governments to set trade, travel and other restrictions aimed at limiting its spread.
Phase 6 is for a full-blown pandemic, characterized by outbreaks in at least two regions of the world.
It could take four to six months before the first batch of vaccines are available to fight the virus, WHO officials said.
Russia, Hong Kong, and Taiwan said they would quarantine visitors showing symptoms of the virus amid global fears of a pandemic, an epidemic spread over a large area, either a region or worldwide.
Mexico canceled school at all levels nationwide until May 6, and the Mexico City government said it was considering a complete shutdown, including all public transportation, if the death toll keeps rising. Labor Secretary Javier Lozano Alarcon said employers should isolate anyone showing up for work with fever, cough, sore throat or other signs of the flu.
Even some of Mexico's most treasured national holidays were affected by the swine flu alert.
