The United States confirmed that it now has 20 cases of swine flu within its borders, including eight new cases among students who attend a New York City high school.

Dr. Richard Besser, acting director of the U.S. Centers for Disease Control, said on Sunday the cases include one in Ohio, two in Kansas, two in Texas and seven in California.

Only one of the U.S. patients had to be hospitalized and all are recovering, Besser told reporters during a news conference.

Besser said U.S. officials expect to find more cases of swine flu as they ramp up surveillance efforts.

"This is moving fast but we view this more as a marathon," Besser said. "We do think that this will continue to spread, but we are taking aggressive action to minimize the impact on people's health."

In response to the new cases, the U.S. Department of Homeland Security declared a public health emergency in order to release federal funds for the effort to stem the virus's spread.

Department of Homeland Security Secretary Janet Napolitano said that the U.S. government will also release 25 per cent of its stockpile, about 12 million doses, of anti-virus medications Tamiflu and Relenza.

Napolitano also said that U.S. officials will begin asking travellers about their health if they are entering the country from areas that have confirmed swine flu cases.

While travellers won't be prevented from entering the country, they may be referred for further testing.

Besser said the strain of swine flu that has been detected in the U.S. matches the strain found in Mexico, where the virus has killed 22 people and may be responsible for more than 60 other deaths.

Officials in Mexico said Sunday that 73 more people have been hospitalized with influenza and authorities are trying to determine if any of them have swine flu.

Earlier Sunday, New York Mayor Michael Bloomberg announced that the CDC confirmed eight cases at St. Francis Preparatory School in Queens, and is still testing specimens from several other students who have fallen ill.

Earlier Sunday, officials said that more than 100 students from the school recently began suffering from fever, sore throat and aches and pains. Some of the students' relatives also fell ill.

"We don't know if this spread will be sustained," Bloomberg said at a news conference. "What's heartening is that the people who have tested positive have only mild illnesses, unlike in Mexico."

Local newspapers reported that some of the students had travelled to Mexico two weeks ago for a spring break trip.

Bloomberg also said that five of six specimens from sick children who attend a daycare in the Bronx tested negative for swine flu, while the sixth was inconclusive and would be re-tested.

Bloomberg said that he has spoken with all New York City hospitals by conference call and they are not reporting an increase in patients suffering from flu-like symptoms, pneumonia or lung infections.

The news comes as ten students from New Zealand are also being tested for swine flu after taking a school trip to Mexico.

All of the patients appear to be recovering well, according to New Zealand Health Minister Tony Ryall, and none have fallen seriously ill.

The patients have all tested positive for influenza but it has not yet been confirmed if they have swine flu, Ryall said.

The 10 students were among 13 who were quarantined and tested upon their return from Mexico. In total, 25 students and teachers arrived back in Auckland on Saturday on a flight from Los Angeles.

One student was hospitalized, said Dr. Julia Peters, Auckland Regional Public Health Services director.

"Ten students have tested positive for Influenza A, and these results will now be sent to the World Health Organization laboratory in Melbourne to ascertain whether it is the H1N1 swine influenza," Peters said.

For now, other passengers who were on the flight are not being checked, according to New Zealand Health Ministry spokesperson Michael Flyger.

Meanwhile, officials in Israel are trying to determine if a man who recently travelled to Mexico and has since been hospitalized with flu-like symptoms has swine flu, while French authorities are investigating four possible cases.

In Spain, three people who recently returned from Mexico were being observed in hospital in the northern Basque region, in southeastern Albacete and the Mediterranean port city of Valencia.

The World Health Organization warned Saturday that the virus is a public health emergency that has "pandemic potential."

More than 1,300 people in Mexico have fallen ill since April 13.

Swine flu, also known as A H1N1, is normally contracted through contact with pigs. However, it appears that this strain is spread through human-to-human contact.

Symptoms include fever, muscle and joint pain, cough, sore throat, eye pain, shortness of breath and extreme fatigue.

To prevent the virus's spread, countries throughout Asia, Europe and the Middle East are warning their citizens against travelling to Mexico and increasing surveillance measures:

• Officials at Tokyo's Narita Airport are taking the temperatures of passengers arriving from Mexico.
• Both Hong Kong and Taiwan said that anyone returning from an affected area with a fever will be quarantined, and the Philippines may follow suit.
• Indonesia is monitoring various entry points for travellers with flu-like symptoms using devices installed to monitor for SARS and bird flu.
• Chinese authorities said anyone suffering from flu-like symptoms who had travelled from swine-flu affected areas must report to local officials.
• Italy, Poland and Venezuela are advising citizens to postpone travel to the affected areas of Mexico and the U.S.
• In Egypt, health authorities are checking about 350,000 pigs in for swine flu.

The WHO has kept its pandemic alert level at phase 3. The organization said the level could be raised to 4 if the virus shows a sustained ability to pass from human to human. Phase 5 would indicate that the virus is found in at least two countries in the same region.

Phase 6 is a full-scale pandemic.

With files from The Associated Press