The White House on Saturday sought to "push resources to the breaking point" as officials tested the government's readiness for a flu pandemic in the United States.

The four-hour drill at the White House complex involved Cabinet secretaries and top federal officials. President Bush, though remaining at the White House for the weekend, did not participate. He went for a bike ride in suburban Maryland on a cold winter morning. Vice President Dick Cheney also stayed away.

White House deputy press secretary Trent Duffy would not discuss the scenario being used to test the government's response plans, such as whether a bioterrorism component was being considered beyond a natural spread of disease.

"It is part of the administration's effort to be ready and have the entire federal government ready to respond in the event of a pandemic," Duffy said. "It's a drill."

Fears of a pandemic have increased in recent months as a virus infecting millions of birds has spread throughout Asia and parts of Europe. While the virus has not yet appeared in the United States, or spread from person to person, officials worry the bird flu could eventually mutate and become as contagious globally as the annual flu.

Human-to-human transmission of the virus would be particularly deadly because humans have no immunity to the virus. So far, the virus has killed about half of the 120 people who have contracted it, all as a result of close contact with poultry.

The administration is working under the worst-case assumption that as many as 90 million Americans would become sick and 2 million would die during a worldwide flu pandemic. A moderate pandemic would kill about 209,000. A severe one, such as the one that occurred in 1918, would kill about 1.9 million people.

To prepare, the administration has been coordinating with Asian nations on methods for early identification of any outbreak, working with the vaccine manufacturing industry to look for ways to speed vaccine production, and stockpiling more Tamiflu, which can reduce the severity of the disease if taken early enough.

Some say that more comprehensive steps are needed to get ready for a possible pandemic, such as boosting the nation's public health infrastructure.