US health officials said on Monday that there are now up to 40 cases of swine flu in five states across the country, and that they had released 25 percent of a federal drugs stockpile to states fighting the pandemic.

Briefing reporters at a news conference in Atlanta on Monday, Richard Besser, acting director of the Centers for Disease Control and Prevention (CDC), said 20 new cases were confirmed due entirely to further testing in New York at a school in Queens, bringing the New York total to 28.

Besser said other cases have been reported in Ohio, Kansas, Texas and California. Of the 40 cases, only one person has been hospitalized and all have recovered.

The best way to keep the disease from spreading, he said, is by taking everyday precautions such as frequent handwashing, coveringup coughs and sneezes and staying away from work or school if not feeling well.

The drug stockpile released amount to "11 million courses of antiviral drugs," he told reporters.

"These are en route to affected states of California, New York and Texas as well as other states around the country," Besser said, adding that US states are also receiving CDC test kits to diagnose swine flu.

Meanwhile, US President Barack Obama on Monday urged Americans to remain calm in the face of a widening outbreak of swine flu, saying that the illness is a "cause for concern" but not alarm.

In remarks delivered at a gathering of the National Academy of Sciences, Obama said that the ailment, which is believed to have killed nearly 150 people in Mexico, "requires a heightened state of alert. But it is 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," the president said.

"I'm getting regular updates on the situation from the responsible agencies, and the Department of Health and Human Services as well as the Centers for Disease Control will be offering regular updates to the American people so that they know what steps are being taken and what steps they may need to take," Obama said.

On Monday, Secretary of Homeland Security Janet Napolitano also told a press briefing that the US government was responding aggressively as if the outbreak would spread into a full pandemic.

"We are proceeding as if we are preparatory to a full pandemic," Napolitano said.

Her briefing came shortly before the World Health Organization raised the severity of its pandemic alert level to four from three on a six-point scale. Level four means there is sustained human-to-human spread in at least one country. Level six is a full-fledged pandemic, an epidemic that has spread to a wide geographic area.

The United States also stepped up checks of people entering the country by air, land and sea and issued a new US travel advisory suggesting "nonessential travel to Mexico be avoided." Napolitano said travel warnings for trips to Mexico would remain in place as long as swine flu is detected.

Besser, acting director of the CDC, warned the public on Monday to brace for more swine flu cases and possibly even death from the killer disease.

"From what we understand in Mexico, I think people need to be ready for the idea that we could see more severe cases in this country and possibly deaths," he said. "That's something people have to be ready for and we're looking for that. So far, thankfully, we haven't seen that. But we're very concerned and that's why we're taking very aggressive measures."

Responding to what some health officials feared could be the leading edge of a global pandemic emerging from Mexico, American health officials declared a public health emergency on Sunday.

At a White House briefing on Sunday, Homeland Security Secretary Janet Napolitano called the emergency declaration "standard operating procedure," and said she would rather call it a "declaration of emergency preparedness."

The emergency declaration was "standard operating procedure," Napolitano said, adding the declaration will "allows us to use medication and diagnostic tests that we might not otherwise be able to use, particularly on very young children, and it releases funds for the acquisitions of additional anti-virals."

Similar emergency declarations were issued for floods in recent months in the US states of Minnesota and North Dakota and President Obama's inauguration in January, she said.

According to the secretary, US officials will begin asking travelers about illness if they are entering the country from areas with confirmed swine flu cases. Passengers won't be barred from getting into the United States. But they could be referred for further testing.

"Travelers who do present with symptoms, if and when encountered, will be isolated per established rules," she said.

"They will be provided both with personal protective equipment and we'll continue to emphasize universal hand washing," she added.

The A/H1N1 swine flu confirmed in the Mexican, US 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.