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

The disease, now being called the 2009 H1N1 flu by U.S. public health officials, 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 in 11 states.

"Every American should know that the federal government is prepared to do whatever is necessary to control the impact of this virus," Obama said, highlighting his request for $1.5 billion in emergency funding for vaccines.

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 Phase 5 — one notch below a full-fledged global pandemic.

"It really is all of humanity that is under threat during a pandemic," WHO Director General Margaret Chan said in Geneva. "We do not have all the answers right now, but we will get them."

Dr. Richard Besser, acting chief of the Centers for Disease Control and Prevention, said in Atlanta there were confirmed cases in 10 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 yesterday that the disease was suspected in 159 deaths, and nearly 2,500 illnesses.

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

BORDER STAYS OPEN

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 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 said.

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 flulike symptoms and 41 of those have been cleared so far. Test results on the other eight were not complete.

Mexico announced it would temporarily suspend all nonessential activity of the federal government and private business from May 1 to May 5. Essential services such as transport, supermarkets, trash collection and hospitals will remain open.

A BOY'S DEATH

Obama offered "my thoughts and prayers" to the family of the Mexican boy who died in Houston.

Health officials in Texas said the child had traveled with his family from Mexico to Brownsville on April 4 and had been sick for five days before being hospitalized there. He then was brought to Houston where he died Monday night.

"This is quite sad news," Besser of the CDC said yesterday. "As a parent, as a pediatrician, I am moved by this and my heart goes out to the family."

Several schools were closed in New York and many more in Texas. Tens of thousands of students were affected in Texas, which also called off all public high school athletic and academic competitions at least until May 11.

The Fort Worth Independent School District said it shut all schools effective tomorrow after one case of swine flu was confirmed at a campus, with three probable cases at three other schools.

The school district, which has 144 schools serving 80,000 students, said in a statement on its Web site that schools will likely not reopen sooner than May 11.

Health and Human Services Secretary Kathleen Sebelius said the school closings, while prudent, also cause "a large ripple effect."

"What happens to the parents? Where do those children go? Do you close the daycare center if a younger sibling is there?" Sebelius said.

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 yesterday 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.

"I want to make it clear. These drugs are effective in treating patients who have acquired the 2009 H1N1 flu virus," Sebelius said on her first full day as the new secretary of Health and Human Services.

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

LATEST CASES

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 yesterday, 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.

Ecuador joined Cuba and Argentina in banning travel to or from Mexico, and Peru banned flights from Mexico. The Panama Canal Authority ordered pilots and other employees who board ships passing through the waterway to use surgical masks and gloves. An average of 36 ships per day pass through the waterway, most from the United States, China, Chile and Japan.

In France, President Nicolas Sarkozy met with Cabinet ministers to discuss swine flu, and the health minister said France would ask the European Union to suspend flights to Mexico.

The U.S., the European Union and other countries have discouraged nonessential travel to Mexico. Some countries have urged their citizens to avoid the U.S. and Canada as well.

Influenza pandemics historically have struck about three times a century. About 40 million people died in the global flu outbreak of 1918-1919, but some 36,000 people die each year in the U.S. of seasonal flu.

Experience suggests that more affluent countries will see milder cases of the disease, while developing countries can expect more severe and deadly cases, said Dr. Keiji Fukuda, the acting WHO assistant director-general.

Of particular concern were the cases in New York City. NYC Health Commissioner Thomas Frieden said yesterday all those with confirmed cases were recovering, including a 19-month-old baby and a young woman who were hospitalized.

McClatchy-Tribune News Service and Bloomberg News Service contributed to this report.