MONDAY, April 27 (HealthDay News) -- The number of confirmed cases of swine flu in the United States has doubled to 40, with all the new cases coming from a New York City high school that had previously reported eight cases of the infectious disease, U.S. health officials said Monday.

The officials also said they were tightening their travel advisory to Mexico -- believed to be the source of the outbreak that continues to reach around the world -- recommending that all nonessential travel to that country be avoided.

And late Monday, the BBC reported that the World Health Organization raised the alert level over swine flu from 3 to 4, two levels shy of declaring a pandemic.

"This situation is evolving very quickly, it is changing quickly," Dr. Richard Besser, acting director of the U.S. Centers for Disease Control and Prevention, said during an afternoon teleconference. "We are officially reporting 40 confirmed cases in the United States. The only change from yesterday is 20 confirmed cases in New York City. These are associated with the same school outbreak and really represent additional testing in that group and not an ongoing spread."

All 40 U.S. patients -- 28 in New York, seven in California, two in Texas, two in Kansas and one in Ohio -- have either recovered or had mild infections, Besser said.

"Later today, we will be releasing a new travel advisory for Mexico," he added. "This is out of an abundance of caution, and we will be recommending that nonessential travel to Mexico be avoided."

Mexico is reporting as many as 1,900 possible swine flu infections and as many as 149 deaths.

Earlier Monday, President Barack Obama said the threat posed by the swine flu outbreak was a cause for concern but "not a cause for alarm."

"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 told a gathering of scientists at the National Academy of Sciences, amid increasing worries worldwide about a possible pandemic, the Associated Press reported.

Besser said that he "expects that we will see [swine flu] cases in other parts of the country, and I would fully expect that we will see a broader range in terms of the severity of infection. Thankfully, so far we have not seen severe disease in this country as has been reported in Mexico."

Besser said the United States will step up checks of people entering the country by air, land and sea, looking for signs of infection, and the CDC will begin distributing "yellow cards at ports of entry."

"These will provide information on swine flu, so that people coming into the United States will have information about this outbreak -- what to do if they become sick, what things they can do to prevent the likelihood that they will become sick," he said.

He also said U.S. officials were questioning border visitors about their health, looking for signs of possible infection.

The fast-unfolding events in the United States -- where all 40 cases have been relatively mild and there have been no deaths -- came in response to some 1,900 swine flu infections and as many as 149 deaths in neighboring Mexico.

Officials in other nations around the globe responded to the threat of a possible pandemic. China, Taiwan and Russia considered quarantines, and several Asian countries scrutinized visitors arriving at their airports, the AP reported.

The European Union on Monday advised against nonessential travel to the United States and Mexico. Early Monday, Spain confirmed that a man hospitalized in the eastern part of the country had tested positive for swine flu, in what's believed to be Europe's first case of the disease. Health authorities were also testing 17 other possible cases in Spain, a major travel link between Mexico and Europe, The New York Times reported.

Responding Monday to the EU's travel advisory, the CDC's Besser said, "Based on the situation in the United States right now, I think it is premature to put travel restrictions on people coming to the United States. As the situation changes, that needs to be evaluated by different countries."

On Sunday, U.S. health officials declared a public health emergency in response to the swine flu outbreak.

Homeland Security Secretary Janet Napolitano said the declaration was a precautionary measure and did not mean that the threat posed by the outbreak was worsening. But, the move allows federal and state governments easier access to flu tests and medications, she said.

Napolitano said the federal government had 50 million doses of the antiviral flu medication Tamiflu, and a quarter of those doses were being released to states, if needed, "particularly prioritizing the states where we already have confirmed incidence of the flu."

On Monday evening, the U.S. Food and Drug Administration, in response to requests from the CDC, issued Emergency Use Authorizations to make diagnostic and therapeutic tools available to public health and medical personnel so they can identify and respond to the swine flu virus under certain circumstances. The agency issued these authorizations for the use of certain Relenza and Tamiflu antiviral products, and for the rRT-PCR Swine Flu Panel diagnostic test.

Napolitano also said Sunday that the Department of Homeland Security had started "passive surveillance protocols to screen people coming into the country."

"All persons entering the United States from a location of human infection of swine flu will be processed by appropriate CDC protocols," she said. "Right now, these are passive. They are looking for people and asking about: 'Are you sick? Have you been sick?' and the like. And if so, they can be referred over for further examination. Travelers who do present with symptoms will be isolated."

Speaking at a Sunday press conference, Dr. Anne Schuchat, the CDC's Interim Deputy Director for Science and Public Health Program, said that U.S. health officials had numerous tools to fight the illness' spread and protect the health of Americans. The swine flu viruses found in the United States are resistant to two antiviral medications -- amantadine and rimantadine -- but are susceptible to the antivirals oseltamivir (Tamiflu) and zanamivir (Relenza), Schuchat said.

Schuchat said steps were already being taken to devise a vaccine against this strain of swine flu, although the process takes time. "We are taking the initial steps in terms of preparing the seed virus to hand off to the industry partners, to produce large quantities. But, you know, it takes months to produce a vaccine," she said.

Besser said there are steps people can take to help prevent catching and spreading the flu, including frequent hand-washing. "If you are sick, it is very important that people stay at home. If your children are sick, have a fever and flu-like illness, they shouldn't go to school. And if you are ill, you shouldn't get on an airplane or another public transport. Those things are part of personal responsibility in trying to reduce the impact," he said.

Meanwhile, in Mexico, authorities continued to take dramatic steps -- including suspending school and public gatherings -- to try to contain the outbreak that officials say has killed as many as 149 people, and sickened more than 1,900 others in that country.

Some of the U.S. cases involved people who had recently returned from trips to Mexico, Schuchat said Sunday. The two cases reported in Kansas involved a husband and wife who had recently been to that country, she said. And The New York Times reported that some of the students at St. Francis Preparatory School, in Queens, had recently come back from Mexico as well.

Also on Sunday, Canadian officials confirmed four "very mild" cases of swine flu at a school in Nova Scotia, and two other cases in British Columbia. According to the AP, a provincial health official said that the infection that sickened the students in Nova Scotia "was acquired in Mexico, brought home and spread."

Swine flu is a respiratory disease of pigs caused by type A influenza. Swine flu does not normally infect humans. However, human infections do occur, usually after exposure to pigs. Symptoms resemble those of the regular flu, including sore throat, coughing and fever.

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