As the death toll climbs in Mexico and suspected cases of the virus doubled stateside, the federal government is advising Americans not to travel to Mexico. Meanwhile, the World Health Organization (WHO) responded by raising its alert level but has yet to declare a global emergency.

The number of U.S. cases rose to 48 after numerous students tested positively in a New York City high school. Other cases have been reported in Ohio, Kansas, Texas and California.

The suspected number of deaths rose to 149 in Mexico, the epicenter of the outbreak with nearly 2,000 people believed to be infected. Worldwide there are 73 cases so far, including six in Canada, one in Spain and two in Scotland.

The WHO raised the alert level to Phase 4, meaning there is sustained human-to-human transmission of the virus causing outbreaks in at least one country.

"At this time, containment is not a feasible option," as the virus has already spread to several other countries, said WHO Assistant Director-General Keiji Fukuda.

Putting an alert at Phases 4 or 5 signals that the virus is becoming increasingly adept at spreading among humans. That move could lead governments to set trade, travel and other restrictions aimed at limiting its spread. Phase 6 is for a full-blown pandemic, characterized by outbreaks in at least two regions of the world.

The European Union health commissioner has already advised Europeans to avoid nonessential travel both to Mexico and parts of the United States.

The U.S. has also stepped up checks of people entering the country by air, land and sea, and the State Department warned U.S. citizens to avoid nonessential travel to Mexico. It said those who live in Mexico should avoid hospitals or clinics there unless they have a medical emergency.

President Barack Obama said the outbreak was reason for concern, but not yet "a cause for alarm."

Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention, said that so far the virus in the United States seems less severe than in Mexico. Only one person has been hospitalized in the U.S.

"I wouldn't be overly reassured by that," Besser told reporters at CDC headquarters in Atlanta, raising the possibility of more severe cases in the United States. "We are taking it seriously and acting aggressively," Besser said. "Until the outbreak has progressed, you really don't know what it's going to do."

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