President Barack Obama has appealed for calm as officials confirmed 40 mild cases of deadly swine flu across the United States and the death toll rose to 149 in Mexico.

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Obama called the outbreak a “cause for concern,” but said it should not provoke alarm, as health officials stressed there had been no new sites of infection found in the past 24 hours.

“The Department of Health and Human Services has declared a Public Health Emergency as a precautionary tool to ensure that we have the resources we need at our disposal to respond quickly and effectively,” Obama said.

World health authorities are anxiously watching the spread of the disease amid fears it could become the next global pandemic.

So far the only deaths have been recorded in Mexico, where the probable toll rose to 149, with 20 people confirmed to have died from the disease.

The United States has recorded the second highest rate of infection, and US officials doubled the number of confirmed US cases to 40 in five states — New York, Ohio, Kansas, Texas and California.

The US Centers for Disease Control and Prevention released 25 percent of a federal stockpile of antiviral drugs to those states fighting the flu, distributing 11 million courses of medication.

Acting CDC director Richard Besser said the numbers had leapt because there were 20 more confirmed cases at a school in New York whose pupils had recently visited Mexico.

“Of the 40 cases, we are only aware of one individual who was hospitalized, and all people who have been infected and were sick have recovered,” Besser told reporters, adding that those affected ranged from seven to 54 years old.

The rapid spread of swine flu around the world, and the deadliness with which it has struck in Mexico, prompted US officials on Sunday to declare a national health emergency.

Homeland Security Secretary Janet Napolitano said US immigration officers would start to screen visitors arriving from infected areas.

“Travelers who do present with symptoms, if and when encountered, will be isolated,” she warned.

Besser said that beyond afflicted US citizens who have visited Mexico, there is only documented case of a secondary infection of swine flu in the United States, “but I wouldn\’t be reassured by that.”

“We\’re seeing significant rates of respiratory infection by contacts (of infected people) and I think some of those individuals will end up testing positive for swine flu virus,” the CDC chief said.

“This virus is acting like a flu virus, and flu viruses spread from person to person.” Besser cautioned Americans against going on “non-essential” travel to Mexico, after Secretary of State Hillary Clinton urged US citizens to exercise caution when visiting the nation at the flu\’s epicenter.

But the CDC official rejected a warning from EU Health Commissioner Androulla Vassiliou that appeared to call on Europeans to avoid travel to the United States and Mexico.

“As the situation changes, then that needs to be evaluated by individual countries,” Besser said.

“Based on the situation in the United States right now, I think it\’s quite premature to put travel restrictions on people coming to the United States.”

There was some comfort that, while the disease has a short incubation period of 24 to 48 hours, no new sites of infection appeared to have been found in the United States in the past 24 hours.

New York Mayor Michael Bloomberg said a total of 28 cases of swine flu had been confirmed at the school in the borough of Queens.

But he said that fears the disease had spread to a second site proved unfounded when tests on six sick people all came back negative.