World Health Organization Director-General Margaret Chan told reporters from the WHO's Geneva, Switzerland, headquarters that the Mexican swine flu cases, as well as eight non-fatal cases detected in the United States, are "a serious situation" that could develop into a pandemic, CNN reported.
Saying it was impossible to predict a pandemic, Chan said nevertheless, "This is an animal strain of the H1N1 virus and it has pandemic potential because it is infecting people."
Swine flu in Mexico City closed schools and other public venues, made nearly 1,000 people ill and killed at least 68 people, public health officials say.
The flu strain appears to be the same as one observed in eight California and Texas cases, the WHO said Friday.
An increasing number of people were wearing face masks in Mexico City as the public was warned to avoid large groups and body contact, including shaking hands and kissing, the Voice of America reported Saturday, adding that the WHO has sent experts to the United States and Mexico to monitor the situation.
New York health officials told CNN Friday they are testing about 75 students at a Queens school for swine flu after the students showed flu-like symptoms this week.