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."