MEXICO CITY — The swine flu epidemic entered a dangerous new phase Monday as the death toll climbed in Mexico and the number of suspected cases there and in the United States nearly doubled. The World Health Organization raised its alert level but stopped short of declaring a global emergency.

The United States advised Americans against most travel to Mexico and ordered stepped up border checks in neighboring states. The European Union health commissioner advised Europeans to avoid nonessential travel both to Mexico and parts of the United States.

The virus poses a potentially grave new threat to the U.S. economy, which was showing tentative early signs of a recovery. A widespread outbreak could batter tourism, food and transportation industries, deepening the recession in the U.S. and possibly worldwide.

The suspected number of deaths rose to 149 in Mexico, the epicenter of the outbreak with nearly 2,000 people believed to be infected.

The number of U.S. cases rose to 48, the result of further testing at a New York City school, although none was fatal. Other U.S. cases have been reported in Ohio, Kansas, Texas and California. 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.

"We are in the most critical moment of the epidemic. The number of cases will keep rising, so we have to reinforce preventative measures," Cordova said at a news conference.

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 4-6 months before the first batch of vaccines are available to fight the virus, WHO officials said.