MEXICO CITY — The swine flu epidemic crossed new borders Tuesday with the first cases confirmed in the Middle East and the Asia-Pacific region, as world health officials said they suspect American patients may have transmitted the virus to others in the U.S.

Most people confirmed with the new swine flu were infected in Mexico, where the number of deaths blamed on the virus has surpassed 150.

But confirmation that people have been infecting others in locations outside Mexico would indicate that the disease was spreading beyond travelers returning from Mexico, World Health Organization spokesman Gregory Hartl told reporters on Tuesday in Geneva.

Hartl said the source of some infections in the United States, Canada and Britain was unclear.

The swine flu has already spread to at least six countries besides Mexico, prompting WHO to raise its alert level on Monday but not call for travel bans or border closings. On Tuesday, countries, including Canada, Israel and France, warned their citizens to avoid nonessential travel to Mexico.

"Border controls do not work. Travel restrictions do not work," Hartl said, recalling the 2003 SARS epidemic that killed 774 people, mostly in Asia, and slowed the global economy. "There was much more economic disruption caused by these measures than there was public health benefit."

Hartl said WHO is advising countries to provide sick people with treatments such as Tamiflu, and make sure national plans are in place to ease the impact of a larger outbreak.

"Governments will need to start thinking about larger-scale care for a specific disease in accident and emergency wards," he said. "Do they have the infrastructure? Do they have the equipment? Do they have the medicines? This is the time, now, to prepare."

WHO raised the alert level to Phase 4, meaning there is sustained human-to-human transmission causing outbreaks in at least one country. WHO's pandemic alert system was revised after bird flu in Asia began to spread in 2004. Monday was the first time it has ever been raised above Phase 3.

Flu deaths are nothing new in the United States or elsewhere. The Atlanta-based Centers for Disease Control and Prevention estimates that about 36,000 people died of flu-related causes each year, on average, during the 1990s in the United States.

But the new flu strain is a combination of pig, bird and human viruses that humans may have no natural immunity to.

Tuesday, Mexico City Mayor Marcelo Ebrard ordered gyms, sports clubs, swimming pools and pool halls closed — extending a growing shutdown that already included schools, state-run theaters and other public places.