Washington - The US Centres for Disease Control and Prevention (CDC) was to issue an advisory against non-essential travel to Mexico - the epicentre of the global swine flu outbreak - acting director Richard Besser said Monday.

"This is out of an abundance of caution," he said, adding that the Atlanta, Georgia-based CDC had confirmed 26 deaths in Mexico were a result of swine flu. "This is a small fraction of what they're seeing," Besser warned.

At least 149 people have died of an influenza-type illness in Mexico, Mexican Health Minister Jose Angel Cordova Villalobos said Monday. An estimated 1,000 people were hospitalized.

Besser said the CDC was working with the World Health Organization and Pan American Health Organization as part of a tri-national team on the ground in Mexico.

Besser confirmed 40 cases of swine flu in the five US states of New York, Ohio, Kansas, Texas and California. "The change is 20 additional cases in New York City. This represents additional testing and not an ongoing spread of the virus."

Those who've fallen ill in the US range in age from 7-54 years. "We've not seen severe disease in this country," Besser said, but cautioned that as the situation was evolving quickly, "you don't know going into an outbreak what it will look like at the end."

US authorities will also be distributing yellow cards at ports of entry with information on swine flu.

Responding to a question on why the outbreak was more severe in Mexico, Besser said, "Why are we seeing a different disease spectrum in Mexico than we're seeing here? I expect we'll see additional cases and spectrum of the disease will expand."