The H5N1 bird flu virus has yet to be detected in North America, but officials fear it is only a matter of time before it crosses the Atlantic after spreading from Asia to Europe and Africa.

While it was impossible to predict the flu's arrival, Leavitt told CNN, "It would not be a big surprise if we saw it even in the next few months."

The United States has stepped up its defenses against the deadly avian flu by awarding 1 billion dollars worth of contracts to five drug companies to develop a vaccine.

The White House released an action plan last week to counter any avian flu pandemic, amid fears that an outbreak could kill as many as 2 million Americans and cause economic havoc.

The U.S. and the European Union will jointly host talks in Vienna on June 6-7 to review strategies to combat the bird flu crisis, including vaccine development.

Vaccine production was a top objective laid out in President George W. Bush's "National Strategy for Pandemic Influenza" released Wednesday.

A total of 206 people worldwide are confirmed by the World Health Organization to have been infected with H5N1, out of which 113 have died.

Leavitt compared such a pandemic to the 1918-1919 influenza epidemic, which killed some 50 million people worldwide.

"There's no reason to believe that the United States will be exempt from that," he said.