The government declared a public health emergency Sunday as the number of confirmed cases in the U.S. rises, and Dr. Richard E. Besser, the head of the U.S. Centers for Disease Control and Prevention, said he expects to see more severe cases.

Besser said health authorities are looking for more cases and finding out more about the strain of swine flu.

Besser assured Americans that health officials are taking action to minimize its impact.

The precautionary step doesn't signal a greater threat to Americans, but it allows the federal and state governments easier access to flu tests and medications.

Homeland Security Secretary Janet Napilotano says roughly 12 million doses of the drug Tamiflu are being released from a federal stockpile of 50 million doses so that states can get it if needed.

Texas Gov. Rick Perry requested more than 37,000 doses on Saturday.

Napilotano said at a White House news conference Sunday that the emergency declaration is standard operating procedure.

One was recently declare for the inauguration and for flooding.

Beser said 20 swine flu cases have now been confirmed in five states including Texas, where two high school students contracted swine flu and a California boy visited before his diagnosis.

That boy’s illness was one of seven confirmed cases in California.

Two cases have been confirmed in Kansas.

One has been confirmed in Ohio.

And New York Mayor Michael Bloomberg said Sunday that the CDC confirmed that eight students at a city high school were infected with swine flu.

Bloomberg stressed that the cases were mild and many are recovering.

The city is awaiting the tests of additional samples to see if more St. Francis Preparatory School students were infected.

About 100 students complained of flu-like symptoms at the school.

Some students went to Cancun on a spring break trip two weeks ago.

Mexico City’s mayor said Sunday five new flu deaths were reported overnight, two of which have been linked to swine flu.

Another 73 people have fallen ill.

Mexican officials say 80 or more people may have been killed by the novel strain, and the virus has likely sickened more than 1,300 since April 13.

In response to the outbreak, Mexico's president has assumed new powers to isolate people infected and inspect homes, incoming travelers and baggage.

Mexican soldiers and health workers are patrolling airports and bus stations in an attempt to isolate those who may have the virus.

Two-dozen new suspected cases were reported in Mexico City alone, where authorities suspended schools and all public events until further notice.

More than 500 events, including concerts and sports games, were canceled in the metropolis of 20 million.

Epidemiologists, however, say Mexico's actions may come too late and detaining the ill may not be enough to contain the outbreak.

The World Health Organization says the outbreak has "pandemic potential," but says it's still too early to tell how far it will spread.

New Zealand's health minister says precautions are being taken after 10 students who just returned from Mexico tested positive for influenza.

Tony Ryall says the cases are "likely" to be swine flu, but there is "no guarantee" the students have the virus.

The group is among 25 high school students and teachers quarantined and tested after returning from a trip to Mexico.

Asian-Pacific nations have heightened their monitoring to be on the alert for possible cases.

Ryall says none of the New Zealanders are seriously ill and seem to be recovering.

At this stage other passengers on the flight with the students were not being sought and the next step would depend on what the tests showed.

Japan's biggest international airport stepped up health surveillance, while the Philippines said it may quarantine passengers with fevers who have been to Mexico.

Symptoms of swine flu, which are similar to those of seasonal flu, include fever, fatigue, lack of appetite and coughing, and in some cases runny noses, sore throats, nausea, vomiting and diarrhea.