Two years ago, the WHO set 2002 as the deadline for getting rid of the virus, amid hopes that the killer disease - officially declared eradicated more than 20 years ago - would never return.

The WHO now believes more research needs to be done into possible new smallpox vaccines and treatment before all smallpox stocks can be safely destroyed.

There are officially only two centres in the world which keep stocks of smallpox virus - the United States Centres for Disease Control and the Russian Vector laboratory.

Meeting at their annual assembly in Geneva, representatives of the WHO's 191 member states approved a recommendation to retain stocks of the smallpox virus.

US assistant surgeon-general Kenneth Bernard told the meeting that research was necessary because the events of 11 September had underscored the lengths to which terrorists were prepared to go.

"We regard the potential release of smallpox as a critical national security issue, not only for us but for the entire world," he said.

Medical success

Smallpox, once a feared disease that claimed millions of lives, kills about a third of its victims and leaves others hideously disfigured.

Its eradication was hailed as one of the world's greatest medical successes.

There is no effective treatment once somebody falls ill, but administering the vaccine in the days after exposure can prevent the disease developing.

However, vaccines cannot be given to people with weakened immune systems, including transplant recipients and people with HIV/Aids.

The WHO's decision comes at the end of a week when delegates discussed at length best way to respond to biological and chemical warfare.

The BBC's Imogen Foulkes in Switzerland says there is clearly concern that public health systems are not fully equipped to cope with a deliberately spread infectious disease, and increased awareness and coordination is needed.