By the end of the simulation, the disease had spread to 25 states and 15 other countries.

The Oklahoma bombing helped spur the US to prepare for attacks at home

The last reported case of smallpox in the US was in 1949, and the country ended its vaccination programmes in 1972.

The nation does not have enough vaccine on hand to deal with a national outbreak, and in the war game, rioting broke out after supplies of the vaccine were exhausted.

After hearing the testimony, Republican Congressman Benjamin Gilman said, "Sadly, events of the last few years, with bombings ... in New York, Oklahoma City, have transformed the bio terrorism debate from the question of 'if' to the seeming inevitability of 'when'. "

Homeland security

The bombing of the Murrah Federal building in Oklahoma City and the World Trade Centre in New York, as well as the sarin gas attack on the Tokyo subway by the Aum Shinrikyo cult helped spur more emphasis on domestic preparedness, said Michael Wermuth, a terrorist expert.

Mr Wermuth heads the Gilmore Commission, which looks at how the US would respond to a terrorist attack involving chemical, biological, radiological or nuclear (CBRN) weapons.

The threat from a CBRN attack come from two sources, either from another country or from a non-state actor such as a terrorist group or a lone wolf such as Timothy McVeigh, he said.

The US has plans in place to deal with such an attack from another country, but it does not have a national plan in place to deal with an attack from a terrorist group at home, Mr Wermuth said.

Fortunately, according to non-classified information, only nation states have the resources for chemical, biological or nuclear weapons.

But "while such an attack is a fairly remote possibility, the consequences of such an attack are of a magnitude we cannot ignore the threat," he said.

Threat exists

And the threat of biological weapons exists both from other states today and, in the future, possibly from terrorist organisations, says Ken Alibek.

A Russian biological weapons expert said many countries are studying such weapons

Mr Alibek was the deputy director of Biopreparat, the civilian arm of the Russian biological weapons programme.

Although the Soviet Union signed the Biological Weapons Convention, it had a high-intensity programme to develop and produce biological weapons duting the 1980s and early 1990s, Mr Alibek said.

The programme employed more than 60,000 people and produced tons of anthrax, smallpox and plague for weapons.

But in the 21st Century, Mr Alibek believes that the real threat will not come from an adversary or enemy capable of starting a total war such as the world wars of the 20th Century.

Instead, "we will see low intensity conflicts and terrorism on different levels," he said, adding that biological weapons presented significant advantages.

"They are cheap, relatively easy to produce and relatively easy to deploy," he said.

US must act

But whether the threat comes from other nations or terrorists, the US must act with respect to the Biological Weapons Convention, according to arms control experts.

Some arms control experts had misgivings about the draft protocol for implementing the ban on biological weapons, including Michael Moodie, the president of the Chemical and Biological Arms Control Institute. "My view is if the protocol was adopted it wouldn't make the world safer," he said.

But he added the worst outcome now is for the US to do nothing. "The US has got to come out with realistic hard-edged package of measures," he said.