"This is out of an abundance of caution," said Richard E. Besser, acting director of the Centers for Disease Control and Prevention.

Late this afternoon, the World Health Organization announced that it was raising its global alert level because the virus is spreading easily from human to human and appears able to cause community outbreaks. The alert level was raised from Phase 3 to Phase 4. The move followed an emergency meeting of a special 15-member panel of experts that advises the agency on the threat level and announcements by Scottish and Spanish health officials of confirmed cases in their countries. The committee had planned to meet tomorrow but moved up the gathering to grapple with the rapid developments.

Besser told a news briefing in Atlanta that the new U.S. cases were all related to an outbreak at a school in New York. The confirmations were the result of additional testing and not a sign that the infection is spreading there, he said. He noted that although 40 cases have been confirmed, most of the cases have been mild. Only three people in the United States have been hospitalized, and all have recovered.

In New York, Mayor Michael R. Bloomberg (I) said the city now has 28 confirmed cases of swine flu and 17 possible cases, all of them clustered at St. Francis Preparatory School in Queens.

Ultimately, Bloomberg told reporters, there may be more than 100 cases from the school, where a small group of students visited Mexico on spring break earlier this month.

The announcements came as the European Union's health commissioner vacillated over whether it was safe to travel to the United States or Mexico. The commissioner, Androulla Vassiliou, initially told reporters in Luxembourg that she was "not worried at this stage" about a global pandemic spreading to Europe but nonetheless urged all travelers to avoid the United States and Mexico "unless it is very urgent for them."

But after U.S. officials objected to that statement and the CDC's Besser said it was "quite premature" for the European Union to tell people not to travel to the United States, Vassiliou backtracked and said she was simply advising Europeans to avoid "unnecessary travel" to areas in North America where there have been "serious outbreaks" of swine flu in humans.

Also today, Spain confirmed the first case of the unusual virus in Europe, heightening fears that an outbreak occurring primarily in Mexico -- and to a much lesser degree the United States -- is spreading to Europe. The patient, a 23-year-old university student who returned from Mexico last week, was being treated in a hospital and his condition was not serious, said Trinidad Jimenez, the Spanish health minister.

Two people who returned home to Scotland from Mexico Friday also were confirmed to have swine flu, Scottish Health Secretary Nicola Sturgeon said. They were expected to recover. British officials are investigating nearly two dozen other ill people to see if they also have swine flu.

At the White House, presidential spokesman Robert Gibbs was peppered with questions today about the death of the director of Mexico's National Museum of Anthropology, Felipe Solís Olguin, who had received President Obama at the museum earlier this month during his visit to Mexico. Solís, 64, was hospitalized two days after the visit with what Mexican newspapers said were flu-like symptoms, and he died April 23.

Gibbs said Obama's doctors have informed him "that the president's health was never in any danger, that he has not exhibited any symptoms" of swine flu. He said nobody who traveled with Obama, including reporters, has shown any symptoms, either.

The White House later issued a statement quoting Ricardo Alday, a spokesman for the Mexican Embassy in Washington, as saying Solís "died of complications of a preexisting condition and not of swine flu." Mexican news media accounts attributed the death to complications of pneumonia and diabetes.

This morning, Obama said his administration was monitoring the swine flu situation closely.

"This is obviously a cause for concern and requires a heightened state of alert," Obama said at an appearance at the National Academy of Science. "But it is not a cause for alarm."

Yesterday, the United States declared a "public health emergency" as authorities from New Zealand to Scotland investigated suspected cases of illness that they feared might be a strain of swine flu that has been identified in Mexico, the United States and Canada.

With the U.S. announcement, civilian and military stockpiles of antiviral drugs were being readied for rapid distribution in the event that transmission of swine flu virus accelerates. The declaration also called for greater vigilance at border crossings and in airports for travelers who are coughing or appear ill.

Those steps fell far short of those that could be invoked in a confirmed pandemic, which could include restricting travel, actively screening travelers for fever or illness, quarantining the sick, closing schools and banning public gatherings.

In Mexico, officials today said the infection is suspected of causing as many as 149 deaths from nearly 2,000 cases of disease. They also announced that schools and nurseries would be closed for a week as health care providers seek to stop the spread of the virus. Masses were canceled yesterday and a high-profile soccer game was played before an empty stadium as officials urged the public to take precautions.

In addition to the cases reported overseas today, suspected cases have been reported in Brazil, New Zealand, France and Israel.

U.S. cases have been reported in California, Texas, Kansas, New York and Ohio. Mexico reported suspected cases in 19 of its 32 states. In Canada, four cases were reported in the Atlantic province of Nova Scotia and two on the Pacific Coast in British Columbia. The American and Canadian cases appeared to generally be milder than the Mexican cases, and none has been fatal.

The A/H1N1 swine flu confirmed in the Mexican, U.S. and Canadian cases is a previously unknown combination of pig, human and avian flu viruses. Pigs, which are easily infected with all three types of flu, can function as "mixing vessels" in which flu viruses exchange genetic material and emerge in new forms.

According to the CDC, people cannot get swine flu by eating pork. However, that did not stop half a dozen countries, including China and Russia, from banning imports of pork and pork products from Mexico and three U.S. states that have reported cases of swine flu.

In response, the U.S. Trade Representative's office issued a statement denouncing the bans, saying they "do not appear to be based on scientific evidence and may result in serious trade disruptions without cause."

In a news conference in New York, Bloomberg said the affected high school in Queens is closed today and Tuesday, and he stressed that there was no evidence the virus had spread beyond the St. Francis students.

"This remains an isolated incident," Bloomberg said, surrounded by public health officials and others. "There is no reason for anyone outside the St. Francis community to stay home." He urged people to go to work and said tourists should continue to visit New York. Bloomberg also said none of the St. Francis cases was serious.

"They are getting the world's best medical care, and we have expectations that they will make full recoveries," Bloomberg said.

"We are going to do everything possible to contain this outbreak," he said. "We're going to get through this. We just have to continue to have patience."

The mayor said all of the ill St. Francis students had been e-mailed a detailed questionnaire asking questions about their symptoms, and he praised the school officials and students for cooperating.

Bloomberg also said there had been an increase in emergency room visits around the city and a rise in the number of people calling city information lines with questions. But so far, he said, this indicated more concern than any actual problem beyond the affected high school.

"What we're seeing is an increase in the number of people worried about the flu," he said.

Of all the ill St. Francis students, Bloomberg said, "they virtually all, with the exception of two, seem to be getting over it in a day or so." He said there were no plans to quarantine the school because "we don't think there's any reason to do it."

The New York City health commissioner, Thomas Frieden, said the city had sent "a small number of additional specimens" from elsewhere around New York to the Centers for Disease Control in Atlanta for testing, but he said there was no evidence the swine flu had spread elsewhere. Tests from a Bronx day care center came back negative for swine flu.

Bloomberg and the health experts at the news conference said it remained a mystery why the flu has appeared far more dangerous in Mexico than the mild cases reported here.

Another unanswered question about the outbreak is whether the virus is still spreading in Mexico.

Richburg reported from New York. Staff writers Sholnn Freeman and Glenn Kessler in Washington, Robin Shulman in New York, Kevin Sullivan in London, Craig Whitlock in Berlin and special correspondent Samuel Sockol in Israel contributed to this report.