Secretary of Homeland Security Janet Napolitano announced the public health emergency in Washington, cautioning Americans not to panic.
"[The emergency declaration] is standard operating procedure and allows us to free up federal, state, and local agencies and their resources for prevention and mitigation; it 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 acquisition of additional antivirals," Napolitano said.
The move deployed some 12.5 million doses of Tamiflu and Relenza, two drugs that appear to have helped those ill with the disease, to areas already reporting cases.
In making the declaration, U.S. officials announced they had uncovered 20 confirmed cases of swine flu in five states -- eight in New York, seven in California, two in Kansas, two in Texas and one in Ohio.
Officials with the U.S. Centers for Disease Control [CDC] said so far the cases in the U.S. have been mild, with only one person requiring hospitalization and none of them proving fatal.
But Dr. Richard Besser, acting director of the CDC, warned the news was likely to worsen in the coming days.
"What we know about this virus is it looks to be the same virus as is causing the situation in Mexico," Dr. Besser said. "And given the reports out of Mexico, I would expect that over time we're going to see more severe disease in this country."
Former officials concurred the news would likely worsen in the coming days.
"You don't know how much it's spread, but you've got to at least make the assumption that there's a lot more virus in this country than is seen at the moment," Jeffrey Koplan, the CDC director from 1998 to 2002, told The New York Times.
Mexico appeared to be the epicenter of the outbreak of the respiratory disease as officials there canceled most public meetings and closed schools in an effort to slow the spread of the disease. In the last two weeks, more than 1,400 people have fallen ill and as many as 86 have reportedly died.
The World Health Organization reported Sunday that they suspected outbreaks of the swine flu in 19 of Mexico's 32 states.
Mexico began seeing a spike in flu cases in mid-March and by early April a number of young adults were becoming ill and dying. Health officials sent samples of the virus to the CDC in Atlanta and a Canadian health lab in Winnipeg.
Scientists decoded the new flu virus and found it contained gene sequences from North American and Eurasian swine flus, North American bird flu and North American human flu.
Already the swine flu has been reported in Canada, Spain and New Zealand.
U.S. officials said they would monitor the border crossings for those appearing to be ill, but had no plans to close the border or implement tighter controls. Other countries have taken much stronger measures. Hong Kong, which grappled with a deadly SARS outbreak in 2003, urged residents not to travel to Mexico and moved to detain potentially ill travelers entering the country. China and Russia also moved to establish quarantines.
The WHO has been monitoring the situation and said it would decide on Tuesday whether to raise the flu pandemic alert level from 3 to 4.
Raising it to level 4 "would be a very serious signal that countries ought to be dusting off pandemic plans," said Dr. Keiji Fukuda, deputy director general of the WHO.