Some New Yorkers are keeping masks by their desks
The WHO has issued a draft report advising governments to start preparing response plans in the event of such an attack, which it said could involve diseases such as anthrax, botulism - which causes paralysis - or smallpox.
"Under these circumstances, it would be imprudent not to be thinking, at least, and planning as hard as possible," he said.
Crop-plane caution
Washington has declared that countries on a US blacklist for sponsoring terrorism have "very active chemical and biological warfare programmes".
Attorney General John Ashcroft said that one of the suspected hijackers, Mohamad Atta, had been gathering information on the use of crop-dusting aircraft, believed to be suitable for a biological attack, before the New York atrocity.
"Our investigation has uncovered several individuals, including individuals who may have links to the hijackers, who fraudulently have obtained or attempted to obtain hazardous material transportation licenses," he said.
Computer disks and hard-drive of one person in custody were discovered to contain information downloaded from the internet on the use of aerial applications of pesticides or crop-dusting, Mr Ashcroft added.
On Tuesday the authorities lifted a ban on the use of crop-duster planes imposed on Sunday, but all law enforcement agencies had been told to remain alert to the threat.
Stocking up
In New York, stores are reported to have sold out of gas masks as frightened residents took steps to protect themselves against such unconventional attacks.
In Britain, army and navy storeowners have also reported that sales of masks and protective clothing have soared in the wake of the attacks.
The WHO says governments are anxious for information.
"We're confident that countries are beginning to look at this as a real possibility because we've had calls from many different health ministers around the world who are concerned," said David Heymann, a WHO official.
"Their concerns are that there will be, or possibly will be, a very serious public health problem in the near future, deliberately caused, and they're asking for guidelines on how to face that situation."
One fear is that people could be infected for several days without knowing it, spreading disease further in the meantime. In such circumstances, experts say, masks would be of little use.