MEXICO CITY — The World Health Organization raised its global alert level on the spreading swine flu virus today, but stopped short of declaring a global emergency — even as the United State said it was acting as if the outbreak would grow into a full pandemic.

The U.S. 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 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 42, 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 coming down with the disease 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 the disease's spread.

The WHO's Phase 6 is the pandemic phase, characterized by outbreaks in at least two regions of the world.

Many experts think it may be impossible to contain a flu virus already spreading in several countries.

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 Barack 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. Only one person has been hospitalized in the U.S.

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

"We are taking it seriously and acting aggressively," Besser said. "Until the outbreak has progressed, you really don't know what it's going to do."

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.

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.

Amid the warnings, the Mexican government grappled with increasing criticism of its response. At least two weeks after the first swine flu case, the government has yet to say where and how the outbreak began or give details on the victims.

The health department lacked the staff to visit the homes of all those suspected to have died from the disease, Cordova said.

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. Of those who have died, 20 have been confirmed to be from swine flu and the government was awaiting results on the others.

As if the country did not have enough to deal with, Cordova's comments were briefly interrupted by a 5.6-magnitude earthquake in southern Mexico that rattled already jittery nerves and sent mask-wearing office workers into the streets of the capital.

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

European Union Health Commissioner Androulla Vassiliou advised Europeans to avoid nonessential travel to Mexico and parts of the United States, although Besser said that including the U.S. in the advisory seemed unwarranted at this time.

State Department spokesman Robert A. Wood said Vassiliou's remarks were his "personal opinion," not an official EU position, and therefore the department had no comment.

"We don't want people to panic at this point," Wood said.

The U.S. stepped up checks of people entering the country by air, land and sea.

Besser said a new U.S. travel advisory was being prepared suggesting "nonessential travel to Mexico be avoided."

The best way to keep the disease from spreading, he 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."

Besser said about 11 million doses of flu-fighting drugs from a federal stockpile have been sent to states in case they are needed. That's roughly one quarter of the doses in the stockpile, he said.

There is no vaccine available to prevent the specific strain now being seen, he said, but some antiflu drugs do work once someone is sick.

If a new vaccine eventually is ordered, the CDC already has taken a key preliminary step — creating what's called seed stock of the virus that manufacturers would use.

Many of the cases outside Mexico have been relatively mild. Symptoms include a fever of more than 100, coughing, joint aches, severe headache and, in some cases, vomiting and diarrhea.

European and U.S. markets bounced back from early losses as pharmaceutical stocks were lifted by expectations that health authorities will increase stockpiles of anti-viral drugs. Stocks of airlines, hotels and other travel-related companies posted sharper losses.

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 flu-like symptoms within two weeks of arrival had to report to authorities.

China, Russia and Ukraine were among the 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.

Germany's leading vacation tour operators were skipping stops in Mexico City as a precaution. The Hannover-based TUI said trips through May 4 to Mexico City were being suspended, including those operated by TUI itself and also through companies 1-2 Fly, Airtours, Berge & Meer, Grebeco and L'tur.

Japan's largest tour agency, JTB Corp., suspended tours to Mexico through June 30.

Russian travel agencies said about a third of those planning to travel to Mexico in early May had already canceled.

———

On the Net:

WHO swine flu page: www.who.int/csr/disease/swineflu/en/index.html

CDC: www.cdc.gov