A nation left jumpy and fearful of a possible next round of terror was not soothed by warnings from bioterror experts speaking to a Senate appropriations subcommittee Wednesday.

George Whiteside, chairman of the Defense Science Board Task Force on Defense Against Chemical Weapons, told the subcommittee that deals with health and human services not to underestimate the threat.

"I would make the point that an issue in biological defense is that there has been a skepticism for some period of time about the seriousness of biological threats — are these truly serious problems?" Whiteside said. "And I would suggest to the committee that in looking at the problem as carefully as we can that there is the potential for very grave harm in biological weapons."

But Secretary of Health and Human Services Tommy Thompson tried to soothe fractured nerves by insisting that federal doctors could quickly contain any bioterrorist attack. He also urged Americans not to be panicked by all the preparations for such a possibility and not to go overboard themselves.

"People should not be scared into believing they need to buy gas masks. And people should not be frightened into hoarding medicine and food," Thompson said. "There is nothing we know of to warrant such actions."

Yet even as Thompson cautioned against stockpiling, he told Americans to be on the lookout for any mysterious symptoms and see a doctor promptly if they have any.

The warnings fell on skeptical ears as some senators said the public couldn't possibly know what symptoms accompany certain outbreaks.

"Few if any of these people have ever seen a case of anthrax or small pox," said Sen. John Edwards, D-N.C. "They don't know what to look for or what the symptoms are and they also don't know how to react."

Thompson, pressed about recent public statements downplaying the threat, insisted his agency could quickly dispatch the proper medical authorities to contain and treat any outbreak from an attack.

"I am absolutely assured we could respond to any contingency," he told senators.

"I just don't believe that," responded Sen. Robert Byrd, D-W.Va., who cautioned against misleading the public.

Thompson acknowledged the health system does have gaps, and said he is seeking $800 million more this year to fill those gaps. Most of the money would go to state and local health departments, to train local physicians and laboratories in recognizing symptoms of anthrax, smallpox and other worrisome agents.

"Public health is a national security issue and must be treated as such," he said in calling for the additional funding.

He received support on that point from Sen. Bill Frist, R-Tenn., a physician.

"Our most significant failure in this country as we look at this coherent strategy of prevention, of preparedness and response, is the lack of investment in our public health infrastructure," he said.

Vaccines against biological agents are not available to civilians today, but are being stockpiled so that if there ever were an attack, people exposed could be vaccinated quickly, Thompson said.

The government has sped up production of 40 million more doses of smallpox vaccine, to be delivered to a national stockpile of 15 million existing doses by late next year instead of in 2005. The nation's only maker of anthrax vaccine, Bioport Corp. of Michigan, had stopped making it for the military after the Food and Drug Administration told it to correct repeated violations of safety regulations. It may be able to resume production of doses for the military within six weeks.

But Thompson's bioterrorism adviser, Dr. Scott Lillibridge, said the vaccines should not be given routinely to the public because it could cause too many side effects, especially since there may never actually be a need for it.

Other Threats to Consider

Whiteside also warned of chemical weapons. He said they are preferable in circumstances such as closed environments where there are large numbers of people.

"Transportation nodes, important buildings and offices and that sort are plausible targets for that type of situation for that kind of attack," he said. "A very serious problem I think are very large quantities of industrial chemicals, which are stored and transported, and which can be in principle hijacked or used by terrorist in a mode vaguely analogous to the aircraft used in the World Trade tower attack."

Larry Wright, chairman of the Defense Science Board Task Force on Defense Information Warfare, added that a more likely attack would be a cyber war.

"For example, in the World Trade Center bombing, if the terrorists had simultaneously taken down the Internet, the cell phone system and the 911 system for the first responders, you can imagine what that would have done and the ability of the first responders to save as many people as they did ... panic that might have ensued in the city," he said.

Fox News' Brian Wilson and the Associated Press contributed to this report.