"We need to do something that even dwarfs the Manhattan project," Frist told the World Economic Forum in Davos. The Manhattan project was the codename for the United States's World War II effort to devise an atomic weapon.

"The greatest existential threat we have in the world today is biological. Why? Because unlike any other threat it has the power of panic and paralysis to be global."

He predicted that the world would experience another bioweapon attack within the next decade, following the limited casualties seen when anthrax was sent through the U.S. mail system in 2001.

Next time, the death rate could be a much, much higher, said Massachusetts Institute of Technology Professor John Deutch.

An attack using the smallpox virus is overwhelmingly the largest risk, he believes.

The disease was officially eradicated three decades ago but Deutch said it was possible former Soviet stocks were still at large or even that small quantities could be extracted from graves.

"Every country has a vulnerability here," he said.

VACCINE

In a bid to protect citizens, the U.S. government has ordered millions of doses of smallpox vaccine as part of a wide-ranging security drive in the wake of the Sept. 11, 2001 attacks.

Other governments are also following suit in stocking up on smallpox shots. But experts warned that other avenues were open to would-be terrorists, with diseases such as plague and Ebola hemorrhagic fever virus options for weaponisation.

More worryingly still, sophisticated groups might in the future use genetic engineering to produce hybrid microbes against which there are no defenses.

Francis Collins, director of the U.S. National Human Genome Research Institute, said such developments raised the question of whether there should be restrictions on publication of some scientific research in biology.

Physicists are already limited from sharing information on atomic weapons technology.

Collins said openness was the best strategy but he suggested there could be specific information about protocols used to create dangerous super-bugs that might, in future, be classified.