President Barack Obama said he was closely watching an outbreak of swine flu in the United States, where 40 cases have been confirmed, and urged people to remain calm.

"This is obviously a cause for concern and requires a heightened state of alert, but it's not a cause for alarm," Obama said.

The government declared a national public health emergency, and the US Centres for Disease Control and Prevention (CDC) and the State Department issued an advisory late Monday urging against non- essential travel to Mexico, in effect through July.

Health experts were struggling to understand swine flu, which has genetic elements that come from three species - pigs, birds and humans - and has never been seen before.

No one has, so far, been seriously ill in the US, and the infections have been self-resolving, with the disease running its course and the patients recovering.

While Mexico is currently believed to be the epicentre of the global swine flu outbreak, the first evidence of swine flu transmission was reported in September in the US state of Texas, involving a young boy who worked with pigs, said Laurie Garrett at the Council on Foreign Relations.

Experts remain unsure how transmissible and lethal the virus is, why there are so many deaths in Mexico and not in the US, whether the swine flu detected last year in Texas was related to the current virus, and why young adults seem to be the most affected.

At least 149 people have died of an influenza-type illness in Mexico, Mexican Health Minister Jose Angel Cordova Villalobos said. The US CDC said that 26 of the Mexican deaths were laboratory- confirmed swine flu cases.

CDC acting director Richard Besser confirmed 40 cases of swine flu in the US states of New York, Ohio, Kansas, Texas and California. Those who've fallen ill in the US range in age from 7-54 years, with a median age of 16 years.

"We've not seen severe disease in this country," Besser said.

He cautioned that the situation was evolving quickly: "You don't know going into an outbreak what it will look like at the end."

In New York City, 28 cases of swine flu were confirmed and another 17 cases were suspected, with more than 100 students at one school becoming sick, Mayor Michael Bloomberg said.

All the New York flu cases are concentrated at a Queens high school, where local news reports said six of the original eight students who reported sick had travelled to Mexico for spring holidays.

Bloomberg said City Hall had contacted all daycare centres in the city of 8 million people and found no case of swine flu, severe illness or possible cases of flu.

"This indicates that so far we have not seen a situation comparable to that of Mexico," Bloomberg said.

In California, where the CDC confirmed seven cases of swine flu, Governor Arnold Schwarzenegger said the state was fully prepared to deal with an outbreak.

He said the state had activated its joint emergency response system, which combines the public health department and the state emergency management agency.

Meanwhile, Homeland Security Secretary Janet Napolitano said the US was working with Mexico and Canada in a "tri-national approach to this emerging flu."

She said the US had no plans to close its borders as that is "something one would do if one had a realistic hope of containment."

At a press briefing she said, "We are simply in preparation mode. We do not yet know how widespread this flu will be within the United States. So we continue to move aggressively to prepare."

Obama's pick for Health and Human Services secretary, Kathleen Sebelius, is still awaiting confirmation in the US Senate. Obama has yet to appoint a CDC director as well as a surgeon general.

US stocks plunged Monday over concerns that the outbreak would hurt the travel, hotel and meat-processing industries, though healthcare stocks rallied.