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WASHINGTON (AP) — President Barack Obama said Monday that the threat of spreading swine flu infections is a cause for concern but "not a cause for alarm" as the United States undertook close border monitoring to contain it.

"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 Monday 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 U.S. officials were questioning border visitors about how their health.

The quickening pace of developments in the United States in response to some 1,600 swine flu infections in neighboring Mexico was accompanied by a host of varying responses around the world. The European Union advised against nonessential travel to the United States and Mexico, China, Taiwan and Russia considered quarantines and several Asian countries scrutinized visitors arriving at their airports.

In the United States, a private school in South Carolina was closed Monday because of fears that young people returning from Mexico might have been infected.

"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."

"But one thing is clear: Our capacity to deal with a public health challenge of this sort rests heavily on the work of our scientific and medical community," the president said. "And this is one more example of why we cannot allow our nation to fall behind."