WASHINGTON — President Barack Obama pledged "great vigilance" in confronting the swine flu outbreak tonight as it spread coast to coast across the U.S.

The outbreak hit 11 states and closed schools amid confirmation of the first U.S. death — a Mexican toddler who visited Texas with his family — and the confinement of dozens of Marines after one came down with the disease in California.

Some 100 schools were closed, and more might need to be shut down temporarily, Obama said, declaring, "This is obviously a very serious situation." The total confirmed cases in the U.S. rose to nearly 100, with many more suspected.

He spoke just hours after the Geneva-based World Health Organization, a United Nations agency, sounded its own ominous alarm, raising its alert level to one notch below a full-fledged global pandemic.

Said WHO Director General Margaret Chan: "It really is all of humanity that is under threat during a pandemic."

Dr. Richard Besser, acting chief of the Centers for Disease Control and Prevention, said in Atlanta there were confirmed cases in ten states, including 51 in New York, 16 in Texas and 14 in California. The CDC counted scattered cases in Kansas, Massachusetts , Michigan, Arizona, Indiana, Nevada and Ohio.

State officials in Maine said laboratory tests had confirmed three cases in that state, not yet included in the CDC count.

Also, Illinois officials cited nine "likely cases," most of them in the Chicago area, and three schools were shut down.

And the Pentagon said a Marine at the Twentynine Palms base in California had been confirmed to be ill with swine flu and was isolated, along with his roommate. A Marine spokesman at the Pentagon, Maj. David Nevers, said the sick Marine was doing well and his condition continued to improve. Nevers said about 30 others who had been in contact with the sick Marine would be held apart for five days as well as to see if they show symptoms.

In Mexico, where the flu is believed to have originated, officials said today that the disease was now suspected in 160 deaths, and nearly 2,500 illnesses.

Despite calls from many U.S. lawmakers for tightening controls over the Mexico-US border, Obama ruled out that option, even though the swine flu outbreak has been at its most virulent and may have begun there.

At a prime-time news conference on his 100th day in office, Obama said he would heed the advice of health officials, to whom blocking the border "would be akin to closing the barn door after the horses are out, because we already have cases here in the United States."

Instead, he said his administration had ramped up screening efforts and made sure needed medical supplies were on hand. "The key now is to just make sure we are maintaining great vigilance, that everybody responds appropriately when cases do come up. And individual families start taking very sensible precautions that can make a huge difference."

He praised the Bush administration for making preparations for such a crisis, including stockpiling 50 million doses of antiviral medications.

"Because this is a new strain, we have to be cautious," Obama said. "If this was a strain we were familiar with, then I think we wouldn't see the kind of alert levels that we're seeing, for instance, with the World Health Organization."

Earlier, Homeland Security Secretary Janet Napolitano said, "Closing our nation's borders is not merited here."

"This virus is already in the United States. Any containment theory ... is really moot at this time," Napolitano said.

In fact, customs agents have delayed 49 people at the border because of flu-like symptoms and 41 of those have been cleared so far. Test results on the other eight were not complete.

The Senate's top Republican said the spread was a "very worrisome situation and we're all following it very closely." Minority Leader Mitch McConnell of Kentucky said Republicans "stand ready to closely work with the administration to protect the American people as this situation unfolds."

Laboratory testing showed the new virus was treatable by the anti-flu drugs Tamiflu and Relenza, and the first shipments from a federal stockpile arrived today in New York City and several other locations. The government was shipping to states enough medication to treat 11 million people as a precaution. All states should get their share by May 3.

No shortages had been reported — there was plenty in regular pharmacies, federal health officials said.

A pandemic is an epidemic that has expanded globally. The swine flu has now been reported on four continents.

Germany and Austria became the latest countries to report infections. Germany reported four cases today, Austria one case.

New Zealand's total rose to 14. Britain had earlier reported five cases, Spain 10. There were 19 cases in Canada and two in Israel.

Of particular concern were the cases in New York City.

Health officials said the number of confirmed swine flu cases in New York had risen to 51, and tests were under way on the first three probable cases outside New York City.

City Health Commissioner Thomas Frieden said today all those with confirmed cases were recovering, including a 19-month-old baby and a young woman who were hospitalized.

Several schools were closed in New York, many more in Texas. Some 53,000 students were affected in Texas.

Obama noted he had asked Congress for $1.5 billion in emergency funds to help build more drug stockpiles and monitor future cases, as well as help international efforts to avoid a full-fledged pandemic, an epidemic that spreads widely across the globe.