A single biological attack on the US could cause 10 times more deaths than a nuclear strike, claims a report from an influential think-tank. The Brookings Institution is advising President Bush to concentrate anti-terrorist efforts on thwarting "doomsday" scenarios such as these.
The report, "Protecting the American Homeland", to be published on Tuesday, estimated that the greatest threat is posed by widely dispersed smallpox, anthrax or ebola.
Doctors expert in the lethal nature and potential spread of such infections helped compile it.
It suggests that a nuclear device exploded in a major US city would kill 100,000 people - but that a million could die if large areas were exposed to lethal bacteria and viruses.
In addition to the death toll, hundreds of billions of dollars worth of economic damage could be caused by biological attack, says the report.
Michael O'Hanlon, from the Brookings Institution, said: "There are an unlimited number of potential vulnerabilities.
"We're going to have to spend some time prioritising and organising our thinking.
"We really should be focusing on potentially catastrophic attacks, meaning large number of casualties or large damage to the economy."
The report urges the government to increase spending on air defences, food safety and cyber-security.
In the wake of the September 11 attacks, the US appointed a Homeland Security Director working to tighten defences against terrorist attacks on American soil.
The White House is seeking a budget of $38bn for homeland security measures in the 2003 budget - a sum described as insufficient by the Brookings report.
Other "vulnerabilities" identified by the report include the possibility of nuclear attacks on the maritime industry using devices concealed in a shipping container.
Attacks of this kind could cost the economy as much as a trillion dollars in losses.
A successful attack on a nuclear or toxic chemical plant could potentially cause 10,000 casualties, said the analysts.
They stressed that biological attack, particularly with Ebola, was a remote possibility.