The declaration, signed Friday night and announced Saturday, comes with the disease more prevalent than ever in the country and production delays undercutting the government's initial, optimistic estimates that as many as 120 million doses of the vaccine could be available by mid-October.

Health authorities say more than 1,000 people in the United States, including almost 100 children, have died from the strain of flu known as H1N1, and 46 states have widespread flu activity. So far only 11 million doses have gone out to health departments, doctor's offices and other providers, according to the Centers for Disease Control and Prevention officials.

Administration officials said the declaration was a pre-emptive move designed to make decisions easier when they need to be made. Officials said the move was not in response to any single development.

Health and Human Services Secretary Kathleen Sebelius now has authority to bypass federal rules when opening alternative care sites, such as offsite hospital centers at schools or community centers if hospitals seek permission.

Some hospitals have opened drive-thrus and drive-up tent clinics to screen and treat swine flu patients. The idea is to keep infectious people out of regular emergency rooms and away from other sick patients.

Hospitals could modify patient rules — for example, requiring them to give less information during a hectic time — to quicken access to treatment, with government approval, under the declaration.

It also addresses a financial question for hospitals — reimbursement for treating people at sites not typically approved. For instance, federal rules do not allow hospitals to put up treatment tents more than 250 yards away from the doors; if the tents are 300 yards or more away, typically federal dollars won't go to pay for treatment.

Administration officials said those rules might not make sense while fighting the swine flu, especially if the best piece of pavement is in the middle of a parking lot and some medical centers already are putting in place parts of their emergency plans.

The national emergency declaration was the second of two steps needed to give Sebelius extraordinary powers during a crisis.

On April 26, the administration declared swine flu a public health emergency, allowing the shipment of roughly 12 million doses of flu-fighting medications from a federal stockpile to states in case they eventually needed them. At the time, there were 20 confirmed cases in the U.S. of people recovering easily. There was no vaccine against swine flu, but the CDC had taken the initial step necessary for producing one.

"As a nation, we have prepared at all levels of government, and as individuals and communities, taking unprecedented steps to counter the emerging pandemic," Obama wrote in Saturday's declaration.

He said the pandemic keeps evolving, the rates of illness are rising rapidly in many areas and there's a potential "to overburden health care resources."

The government now hopes to have about 50 million doses of swine flu vaccine out by mid-November and 150 million in December. The flu virus has to be grown in chicken eggs, and the yield hasn't been as high as was initially hoped, officials have said.

"Many millions" of Americans have had swine flu so far, according to an estimate that CDC Director Dr. Thomas Frieden gave Friday. The government doesn't test everyone to confirm swine flu so it doesn't have an exact count. He also said there have been more than 20,000 hospitalizations.