UPDATE 2-WHO chief says swine flu has pandemic potential

* WHO emergency committee to advise on changing alert level

* All countries urged to be vigilant for unusual cases

* Premature to issue travel advisories, WHO head says

* Too soon to switch to production of new flu vaccine

(Adds details, quotes, byline)

By Stephanie Nebehay

GENEVA, April 25 (Reuters) - Outbreaks of swine flu in Mexico and the United States have the potential to cause a worldwide pandemic but it is too early to say whether they will, the head of the World Health Organisation said on Saturday.

WHO director-general Margaret Chan urged health authorities in all countries to be on high alert for unusual patterns of disease and any rise in severe flu or pneumonia cases.

"This is clearly an animal strain of the H1N1 virus and it has pandemic potential because it is infecting people," Chan said on a teleconference.

"However, we cannot say on the basis of currently available laboratory, epidemiological, and clinical evidence whether or not it will indeed cause a pandemic."

The United Nations health agency has warned for several years that a new virus strain could spark a human influenza pandemic that could sweep around the globe and kill millions.

The new H1N1 flu strain -- a mixture of swine, human and avian flu viruses which has killed up to 68 people among 1,004 suspected cases in Mexico and infected eight in the United States -- is still poorly understood and the situation is evolving quickly, Chan said.

There were currently no indications of similar outbreaks elsewhere in the world, she said.

"It would be prudent for health officials within countries to be alert to outbreaks of influenza-like illness or pneumonia, especially if these occur outside in months outside the usual peak influenza season," added Chan, a former health director of Hong Kong.

Health officials worldwide should also be alert to large incidences of severe or fatal flu-like illness in groups other than young children and the elderly, the ages usually at highest risk from normal seasonal flu, she said.