Richard Besser, acting head of the U.S. Centers for Disease Control and Prevention, revealed that American authorities were undertaking "passive screening" at its borders and reiterated the Obama administration's call for people to remain calm. Besser said that U.S. officials at border checkpoints were "asking people about fever and illness, looking for people who are ill."

Besser discussed the problem on morning news shows as President Barack Obama prepared to address it later Monday morning in remarks to a meeting of the nation's top scientists.

The U.S. declared a national health emergency Sunday in the midst of uncertainty about whether a mounting sick count really meant ongoing infections - or just that health officials had missed something simmering for weeks or months. But the declaration did allow Washington to ship roughly 12 million doses of flu-fighting medications from a federal stockpile to states in case they eventually need them.

Besser traveled the morning news-show circuit Monday, telling interviewers the U.S. government was being "extremely aggressive" and saying he wouldn't personally recommend traveling to parts of Mexico where the new virus has taken hold. But he noted that the issue of a travel ban was under discussion and that nothing had been decided.

Besser said he was not reassured by the fact that so far in the U.S., no one has died from the disease.

"From what we understand in Mexico, I think people need to be ready for the idea that we could see more severe cases in this country and possibly deaths," he said. "That's something people have to be ready for and we're looking for that. So far, thankfully, we haven't seen that. But we're very concerned and that's why we're taking very aggressive measures."

A private school in South Carolina closed after some students returned from Mexico with flu-like symptoms.

Officials of Newberry Academy said in a statement that seniors were in Mexico earlier this month and some had flu like symptoms when they returned. Calls to the school went unanswered Monday.

State Department of Health and Environmental Control spokesman Jim Beasley said test results on the students could come back as early as Monday afternoon. The agency has stepped up efforts to investigate all flu cases in South Carolina. There have been no confirmed swine flu cases in the state.

In Mexico, the outbreak's epicenter, soldiers handed out 6 million face masks to help stop the spread of the novel virus that is suspected in up to 103 deaths. Most other countries are reporting only mild cases so far, with most of the sick already recovering. Cases have been confirmed in Canada - six - and the U.S. - 20.

Spain reported its first confirmed swine flu case on Monday and said another 17 people were suspected of having the disease. The European Union health commissioner advised Europeans to avoid nonessential travel to Mexico and the United States. Also, three New Zealanders recently returned from Mexico are suspected of having it.

Elsewhere, the European Union advised against nonessential travel to Mexico and the United States. China, Russia and Taiwan considered quarantines, and several other Asian countries undertook to question travelers arriving at airports.

Complicating response strategies across the globe was what World Health Organization spokesman Peter Cordingley described as major difficulty that experts were having in assessing precisely the nature of the threat.

"These are the early days. It's quite clear that there is a potential for this virus to become a pandemic and threaten globally," Cordingley said. He said it was spreading rapidly in Mexico and the southern United States.

Cordingley said "honestly don't know" the extent of the problem. He added: "We don't know enough yet about how this virus operates. More work needs to be done."

Multiple airlines, including American, United, Continental, US Airways, Mexicana and Air Canada, said they were waiving usual penalties for changing reservations for anyone traveling to, from, or through Mexico, but have not canceled flights.

Meantime, the World Bank said it would send Mexico $25 million in loans for immediate aid and $180 million in long-term assistance to address the outbreak, plus advice on how other nations have dealt with similar crises. Mexico officials say the flu strain may have sickened 1,614 people since April 13 but laboratory testing to confirm that and how many truly died from it - at least 22 so far out of the 103 suspected deaths - is taking time.

Worldwide, attention focused sharply on travelers.

"It was acquired in Mexico, brought home and spread," Nova Scotia's chief public health officer, Dr. Robert Strang, said of Canada's first confirmed cases.

A New York City school where eight cases were confirmed will be closed Monday and Tuesday, and 14 schools in Texas, including a high school where two cases were confirmed, will be closed for at least the next week. Some schools in California and Ohio also were closing after students were found or suspected to have the flu.

In additions to preparations for quarantines in Russia, Taiwan and China, Singapore, Thailand, Japan, Indonesia, South Korea and the Philippines were checking for signs of fever among passengers arriving at airports from North America. In Malaysia, health workers in face masks took the temperatures of passengers as they arrived from a flight from Los Angeles.

Travelers with flu-like symptoms would be given detailed health checks.

Vesser said that while the U.S. hasn't advised against travel to Mexico, it has urged people to take precautions, such as frequent hand-washing while there.