By CAM SIMPSON

WASHINGTON — Federal officials declared a health emergency on Sunday, as they confirmed 20 cases of swine flu in five states, mobilized medicine stockpiles, and warned the public that the number and severity of incidences were likely to increase.

The officials urged calm, saying the declaration was largely meant to free up resources and precautionary in nature. Homeland Security Secretary Janet Napolitano, appearing with other officials at a news conference at the White House on Sunday, compared it to the kind of alert that goes up before officials know how severe a gathering storm might become.

“We’re responding aggressively to try and learn more about this outbreak and implement measures to control this outbreak,” said Dr. Richard Besser, acting chief of the Centers for Disease Control and Prevention.

The White House briefing Sunday appeared aimed at showing the Obama administration was on top of the situation, as some other countries around the world also reported cases of swine flu. U.S. officials said President Barack Obama is monitoring developments “very closely” and receiving regular updates and briefings.

Symptoms of the new variant of the disease, a combination of pig, bird and human viruses that humans may have no natural immunity to, appear similar to those of regular seasonal flu, such as fever, headache, cough and sore throat, body aches, fatigue and chills. Severe cases of swine flu can lead to death.

In Mexico, 81 people have died from what is believed to be swine flu, with 20 cases confirmed. Canada has confirmed four cases, while Israel and Spain have reported suspected cases. On Saturday, CDC confirmed that eight high school students in New York have been infected with swine flu. The World Health Organization declared the strain a “public health emergency of international concern.”

Dr. Besser said the 20 U.S. cases confirmed to date were all mild, with every patient recovering and only one requiring hospitalization. But he predicted that additional cases could be found, in part because public health officials nationwide are now aggressively searching for those affected by the disease and testing people who show symptoms.

“I would expect over time we’re going to see more severe disease in this country,” Dr. Besser said. The states with confirmed cases are New York, California, Texas, Kansas and Ohio.

Ms. Napolitano said border guards and other security workers at the nation’s ports of entry were screening people coming into the U.S. and that anyone showing signs of illness could be questioned or tested, if necessary. She said more aggressive measures could be introduced in the future, but weren’t needed now.

Ms. Napolitano said the federal government was releasing 12.5 million courses of its emergency stockpile of potentially effective antiviral drugs, leaving 37.5 million courses available. Dr. Besser said the disease has proven “susceptible” to these drugs, but added that officials were working to determine their ultimate effectiveness against the new swine flu strain.

Officials said they didn’t believe the outbreak is related to bioterrorism.

White House spokesman Robert Gibbs said Mr. Obama, who recently returned from a trip to Mexico, hasn’t been affected by the disease. “There’s no reason to believe that his — or anyone who traveled with him — health was ever in jeopardy,” Mr. Gibbs said. U.S. officials said they were coordinating with Mexican and Canadian health officials, as well as officials from international health organizations.

The U.S. officials said people should practice common sense, such as frequent hand washing, and anyone who felt sick should stay home. Dr. Besser suggested that schools that identify cases should close, until authorities get a better handle on the spread of the disease.

WHO Director-General Margaret Chan warned that the virus had the potential to cause a pandemic, but cautioned that it was too early to tell whether it would erupt into a global outbreak. On Sunday, she held teleconferences with staff and flu experts around the world but stopped short of recommending specific measures to halt the disease beyond urging governments to step up their surveillance of suspicious outbreaks.

Following an emergency meeting on Saturday, a WHO panel declared the developments thus far a public health emergency and urged governments around the world to intensify surveillance for unusual outbreaks of flu-like illness and severe pneumonia. But the panel held off on raising a global pandemic alert, saying it needed more information before making a decision.

Food and Drug Administration leaders began holding discussions on the outbreak with the Centers for Disease Control on Thursday, an FDA official said. Acting FDA Commissioner Joshua Sharfstein named the FDA’s chief medical officer, Jesse Goodman, to direct the FDA’s swine flu efforts. Dr. Goodman has been director of the FDA division that handles biologics, highly complex drugs derived from human cells. Vaccines are biologics.

Dr. Goodman is coordinating with the CDC on testing and treatment. Cooperation between the CDC and FDA is crucial to controlling this outbreak, said former Deputy FDA Commissioner Scott Gottlieb, who was at the agency during 2006’s avian flu scare. Mr. Gottlieb said any tension between the FDA and CDC staff could stall the approval and implementation of diagnostic tests in the field, such as the tests that are used in young children.

The FDA hasn’t formally approved any tests for swine flu, but the CDC is ready to use one version of a test that is accepted by medical experts. The FDA and CDC were also developing federal health “emergency use authorization” guidelines over the weekend, which could protect doctors or institutions using flu medicine on very young children from liability issues.

The FDA’s incoming commissioner, Margaret “Peggy” Hamburg, is a bioterrorism expert and was the New York City public health commissioner during an outbreak of tuberculosis. But she has yet to be confirmed by the Senate.

Governments world-wide stepped up surveillance for the deadly virus. Nova Scotia public health official confirmed four cases of swine flu in the Canadian province Sunday. New Zealand said 10 students “likely” had been infected with swine flu after a school trip to Mexico. The Israeli Health Ministry said there is one suspected case in the country.

Spain’s Health Ministry said three people who just returned from Mexico were under observation. French Health Ministry officials said tests on four possible cases of swine flu came back negative. Officials at Tokyo’s Narita airport installed a device at the arrival gate for flights from Mexico to measure the temperatures of passengers. Hong Kong and Taiwan said visitors to infected areas who have fevers would be quarantined — a precaution that the Philippines is also considering. Hong Kong also joined South Korea in warning against travel to Mexico.

Indonesia has increased surveillance at all entry points for travelers with flu-like symptoms, using devices at airports that were put in place years ago to monitor for severe acute respiratory syndrome, or SARS, and avian flu. It said it was ready to quarantine suspected victims if necessary.

New York City health officials said that despite some students testing positive for the swine flu virus at a Queens preparatory school, there appeared to be no citywide spread of the illness, nor additional clusters. CDC tests confirmed cases of human swine flu among students at St. Francis Preparatory School in Queens. The city reported that the school would suspend classes on Monday.

City officials said Sunday afternoon that the flu strain that sickened students was same strain as one found in Mexico. However, the affected students have experienced only mild symptoms and many are already improving, health officials said.

The city health department’s surveillance system has not shown a citywide increase in flu-like illness. An investigation of a cluster of children with illness in a Bronx day-care facility has so far not identified any confirmed or probable cases, according to the city health department.

The health department said the Queens investigation began last week, when more than 100 students at St. Francis developed flu-like symptoms, including fever and sore throat. The department’s Public Health Laboratory tested nine nose and throat swabs. Eight tested positive for Influenza A but didn’t match any of the known human variants of that virus by available testing methods.

On Saturday, the city health department labeled them “probable” cases of human swine influenza and sent the samples to the CDC in Atlanta for confirmatory testing. Those tests confirmed the presence of swine influenza viruses.

—Henry Pulizzi, Alicia Mundy and the Associated Press contributed to this article.

Write to Cam Simpson at •••@••.•••