The World Health Organization says some students in New York infected with swine flu had not traveled to Mexico and must have contracted the disease from returning classmates.

That is significant because it suggests the swine flu virus that is suspected in dozens of deaths in Mexico is now strong enough to be passed among people in other countries, raising the likelihood of a flu pandemic.

There had been suspicions that the diseases was being passed from human to human outside of Mexico, but this is the first confirmation.

WHO flu chief Dr. Keiji Fukuda said Monday that global health experts are examining the situation closely. But he said it was still too early to say if a pandemic that threatens the entire world is "inevitable."

