WASHINGTON D.C., (AP):

President Barack Obama said yesterday that the threat of spreading swine flu infections is a cause for concern but "not a cause for alarm", while customs agents began checking for illness in people traveling to the United States by land and air.

The US has declared a national health emergency in the midst of uncertainty about whether the mounting sick count - 1,600 or more in Mexico alone - was from ongoing infections or merely resulted from health officials missing something that had been simmering for weeks or months.

No fatal cases

The World Health Organisa-tion said yesterday there are now 40 confirmed cases in the US, twice the number previously reported by the Centers for Disease Control and Prevention in Atlanta. None of the cases in the US has been fatal, the organisation said.

Obama said the declaration of an emergency was to make sure the US was prepared.

"The Department of Health and Human Services has declared a public-health emergency as a precautionary tool to ensure that we have the resources we need at our disposal to respond quickly and effectively," Obama told a gathering of scientists, amid increasing worries worldwide about a possible pandemic.

The acting head of the Centers for Disease Control and Prevention earlier yesterday had said that people should be prepared for the problem to become more severe, and that it could involve "possibly deaths". Dr Richard Besser said US officials were questioning border visitors about their health.

Schools closing

A private school in South Carolina was closed yesterday because of fears that young people returning from Mexico might have been infected. A New York City school, where eight cases were confirmed, was clossed yesterday and will be closed today, 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.

"We are closely monitoring the emerging cases of swine flu in the United States," Obama said. "I'm getting regular updates on the situation from the responsible agencies, and the Department of Health and Human Services, as well as the Centers for Disease Control will be offering regular updates to the American people so that they know what steps are being taken and what steps they may need to take."

US customs officials began checking people entering US territory. Officers at airports, seaports and border crossings were watching for signs of illness, said Customs and Border Protection spokesman Lloyd Easterling.