(AP) - With at least 50 cases of swine flu confirmedin the U.S., the Obama administration said Monday it was respondingaggressively as if the outbreak would spread into a full pandemic.

Officials urged Americans against most travel to Mexico as thevirus that began there spread to the United States and beyond.

President Barack Obama urged calm, saying there was reason forconcern but not yet "a cause for alarm."

Dr. Richard Besser, acting director of the Centers for DiseaseControl and Prevention, said that so far the virus in the UnitedStates seems less severe than in Mexico. Only one person has beenhospitalized in the U.S.

Yet just in case, administration officials said that they werealready waging a vigorous campaign of prevention, unsure of theoutbreak's severity or where it would show up next.

U.S. customs officials began checking people entering U.S.territory. Millions of doses of flu-fighting medications from afederal stockpile were on their way to states, with priority givento the five already affected and to states along the Mexicanborder.

The World Health Organization raised the severity of its pandemic alert level to four from threeon a six-point scale. Level four means there is sustainedhuman-to-human spread in at least one country. Level six is afull-fledged pandemic, an epidemic that has spread to a widegeographic area.

Besser said that so far the disease in the United States hasseemed less severe than the outbreak in Mexico, where more than1,600 cases had been reported and where the suspected death tollhad climbed to 149. No deaths had been reported in the U.S.

The number of confirmed cases more than doubled on Monday.Besser said this was due to further testing - not further spreadingof the virus - in a New York City school. Officials have reported48 U.S. cases: 28 in New York, 11 in California, six in Texas, twoin Kansas and one in Ohio.