The US administration sent mixed signals on the threat from bird flu on Thursday, with President George W. Bush urging mass production of vaccines while his health secretary played down the risk of a pandemic.
All officials conceded the United States was unprepared for a possible pandemic, and pointed to a number of meetings being held this week to confront the problem.
The White House said Bush would meet US manufacturers on Friday and urge them to come up with ways to mass produce a vaccine for the H5N1 avian influenza virus.
The virus has killed or forced the destruction of tens of millions of birds and infected more than 100 people, killing at least 60 in four Asian nations since late 2003.
The head of the US Centers for Disease Control and Prevention has said an influenza pandemic that could kill millions is certain and may be imminent.
However US Health and Human Services Secretary Michael Leavitt, while urging preparations for a possible outbreak, said the risk was relatively low and a pandemic probably would not happen.
"The probability that we'll have a pandemic flu is unknown," Leavitt said at a Washington health technology conference. "I will tell you from all I hear from scientists and physicians it is relatively low, but it is not zero."
The risk is high enough that the United States should be prepared, he added. And it is not. "Here's the dilemma: we're not prepared as a country. No one is prepared in the world. We're not alone in this," Leavitt said.
"H5N1 may happen, but it probably won't. If it does we need to be better prepared."
Bush, whose standing was hurt by the slow federal reaction to Hurricane Katrina last month, appeared determined to scotch criticism that he is acting too slowly over the flu threat.
The United States is preparing to open an international meeting of top officials about the virus and Congress is debating whether the country is ready to handle an epidemic.
Experts say the H5N1 strain is mutating steadily and fear it will eventually acquire the changes it needs to easily infect and spread among humans. If it does, they say, it will sweep around the world in months, possibly killing millions.
The World Health Organisation has been tracking the virus, taking samples and sending them to labs to be tested for mutations. WHO officials say quick action will be needed if the virus does make the final jump to become a human infection.
Some experts say the virus could theoretically be contained if the first human victims of a new strain are quarantined, treated quickly with antiviral drugs, and others around them vaccinated.
But stocks of antiviral drugs, such as Gilead and Roche's Tamiflu, are limited, and the companies do not have the capacity to make large quantities quickly.