The swine flu virus that is responsible for an outbreak in Mexico and has been detected in the southwestern United States has "pandemic potential," a top international health official warned Saturday.

"It has pandemic potential," Margaret Chan, the director-general of the World Health Organization, told reporters during a telephone briefing. "It is infecting people."

Chan held the briefing after cutting short a trip to the United States so she could rush back to the WHO's headquarters in Geneva to convene an emergency meeting of special experts to decide what steps should be taken to contain the virus. It is the first time the committee has been called upon since it was created two years ago to handle disease outbreaks.

"The situation is evolving quickly," said Chan. "We do not yet have a complete picture of the epidemiology or the risk, including possible spread beyond the currently affected areas. Nonetheless, in the assessment of WHO this is serious situation that must be watched very carefully."

The WHO could recommend declaring an international public health emergency, which could have major global implications on travel and trade. It could also raise its "pandemic alert level," which is currently at level three. That level means very limited spread of virus from person to person.