Responding to what some health officials feared could be the leading edge of a global pandemic emerging from Mexico, American health officials declared a public health emergency on Sunday as 20 cases of swine flu were confirmed in this country, including eight in New York City.

Other nations imposed travel bans or made plans to quarantine air travelers as confirmed cases also appeared in Mexico and Canada and suspect cases emerged elsewhere.

Top global flu experts struggled to predict how dangerous the new A (H1N1) swine flu strain would be as it became clear that they had too little information about Mexico’s outbreak  in particular how many cases had occurred in what is thought to be a month before the outbreak was detected, and whether the virus was mutating to be more lethal, or less.

“We’re in a period in which the picture is evolving,” said Dr. Keiji Fukuda, deputy director general of the World Health Organization. “We need to know the extent to which it causes mild and serious infections.”