"We do not yet have a complete picture of the epidemiology or the risk, including possible spread beyond the currently affected areas," Chan said. "Nonetheless, in the assessment of WHO, this is a serious situation."

How the situation will evolve is "unpredictable," she said, urging other countries to "increase vigilance."

The Mexican government reported 20 deaths from the disease and it may be responsible for dozens of other deaths.

The new flu strain -- a mixture of swine, human and avian flu viruses has killed 68 people among 1,004 suspected cases in Mexico and infected eight in the United States. But the new virus is still poorly understood and the situation is evolving quickly, Chan warned.

-- Click for more news in Tech Max >>

Editor:Yang Jie