MEXICO CITY — The swine flu epidemic entered a dangerous new phase yesterday 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. European Union health commissioner Androulla Vassiliou 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 152 in Mexico, the epicenter of the outbreak with nearly 2,000 people believed to be infected.

The number of U.S. cases rose to 50, 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 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.

"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.

QUARANTINE PLANS

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.

President Obama said the outbreak was reason for concern, but not yet "a cause for alarm."

Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention, said that so far the virus in the United States seems less severe than in Mexico.

"I wouldn't be overly reassured by that," Besser told reporters at CDC headquarters in Atlanta, raising the possibility of more severe cases in the United States.

U.S. customs officials began checking people entering U.S. territory. Millions of doses of flu-fighting medications from a federal stockpile were on their way to states, with priority given to the five already affected and to border states. Federal agencies were conferring with state and international governments.

"We want to make sure that we have equipment where it needs to be, people where they need to be and, most important, information shared at all levels," said Janet Napolitano, head of the Homeland Security Department.

"We are proceeding as if we are preparatory to a full pandemic," Napolitano said.

She said travel warnings for trips to Mexico would remain in place as long as swine flu is detected.

SCHOOLS CLOSED

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.

Cordova said 1,995 people have been hospitalized with serious cases of pneumonia since the first case of swine flu was reported April 13. The government does not yet know how many were swine flu.

Aside from the confirmed cases, 13 are suspected in New Zealand, and one is suspected in both France and Israel.

The State Department said those who live in Mexico should avoid hospitals or clinics there unless they have a medical emergency.

The best way to keep the disease from spreading, Besser said, is by taking everyday precautions such as frequent handwashing, covering up coughs and sneezes, and staying away from work or school if not feeling well. He said authorities are not recommending that people wear masks at work because evidence that it is effective "is not that strong."

WHO spokesman Peter Cordingley singled out air travel as an easy way the virus could spread, noting that the WHO estimates that up to 500,000 people are on planes at any time.

Governments in Asia — with potent memories of previous flu outbreaks — were especially cautious. Singapore, Thailand, Japan, Indonesia and the Philippines dusted off thermal scanners used in the 2003 SARS crisis and were checking for signs of fever among passengers from North America. South Korea, India and Indonesia also announced screening.

In Malaysia, health workers in face masks took the temperatures of passengers as they arrived on a flight from Los Angeles.

China said anyone experiencing flulike symptoms within two weeks of arrival had to report to authorities.

China, Russia and Ukraine were among countries banning imports of pork and pork products from Mexico and three U.S. states that have reported swine flu cases, while other countries, such as Indonesia, banned all pork imports.

The CDC says people cannot get the flu by eating pork or pork products.