The White House is hosting a top-level exercise this weekend to test the federal government's plans for responding to any flu pandemic outbreak in the United States.

The four-hour "tabletop exercise" is being held Saturday at the White House and will be attended by Cabinet secretaries and other top government officials, but not the president, White House press secretary Scott McClellan said Wednesday.

"We have done much to plan for a pandemic," McClellan said. "But planning alone is not enough. Plans must be tested and improved upon."

McClellan did not say whether state officials also would participate, or give any other details of how the exercise would be conducted.

Fears of a pandemic have increased in recent months as a virus infecting millions of birds has spread throughout Asia. While the virus has not yet spread from person to person, officials fear that it could eventually mutate and become as contagious 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 as a result of close contact with poultry.

The administration is working under the assumption that as many as 90 million Americans would become sick during a global 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.