WASHINGTON - Fearing a biological attack from Iraq, the United States has made new smallpox vaccine and readied response plans for the reappearance of the deadly virus, which some experts are convinced is part of Saddam Hussein's arsenal.

Clues include United Nations weapons inspectors' discovery in 1994 at an Iraqi medical complex of a freeze-dryer labeled "smallpox" in Arabic and Iraq's experimenting with a related virus that infects camels.

The official U.S. position, shared by some experts, is that the evidence is inconclusive.

"I don't believe the intelligence community has a smoking gun that Iraq possesses the virus," said Jonathan Tucker, a former U.N. biological weapons inspector.

"My impression is they're erring on the side of caution on these bits of circumstantial evidence that are troubling but not conclusive."

All smallpox samples except one in Atlanta and one in Moscow were supposed to have been destroyed more than two decades ago when the disease was declared eradicated.

The former Soviet Union reportedly grew tons of smallpox as part of its biological weapons program, and experts suspect other countries could be harboring clandestine samples.

Chief among the suspects is Iraq, which has admitted making weapons from anthrax and other germs.

"I have no doubt in my mind that Iraq does have the smallpox virus," said Dr. Ken Alibek, a top official in the Soviet biological weapons program before he defected to the United States in 1992.

An attack with smallpox could kill millions, a possibility that prompted the Bush administration to order enough smallpox vaccine to inoculate the entire U.S. population if necessary.

"We're very worried about Iraq," said Dr. D.A. Henderson, a smallpox expert and bioterrorism adviser to the Department of Health and Human Services.

Unlike anthrax, the bacteria used in last year's unsolved mail attacks, the highly contagious smallpox virus can be passed from person to person.

The virus causes ugly pustules on the skin and inside the mouth and throat. About a third of unvaccinated people who get the disease die.

The last case of smallpox in the United States occurred in Texas in 1949, and routine vaccinations ended in America 30 years ago.

That means at least two out of five Americans have not been vaccinated, and studies suggest that the vaccine's protection probably fades over time.