It says it fears the disease could be used in a terrorist attack or may revive itself naturally.

Only the US and Russia are known to have samples of smallpox, but there are fears that terrorist organisations have got hold of some.

Russia is also opposed to destroying its stocks.

Disease was eradicated

The last outbreak of the disease occurred in Somalia in 1977.

The World Health Organisation (WHO) declared the disease eradicated in 1980 and set a June 1999 date for the destruction of all remaining samples in laboratories.

Smallpox causes pus-filled blisters all over the body and kills around 20% of those infected.

A study published by the US National Academy of Science's Institute of Medicine last month recommended that surviving laboratory specimens of smallpox be kept alive as a safety net in case the disease breaks out again.

The disease was eradicated following a worldwide vaccination programme, but - in the event of its re-emergence - remaining stocks of the vaccine could treat only about 6 million people.

But the study pointed out that much of the world's population is no longer inoculated against the disease.

The virus itself is essential to production of the vaccine.

The US also wants to keep samples of the virus because scientists say its genetic material could be useful in developing new drugs and vaccines.

Governments fear bioterrorism

Stocks of the vaccine would be essential in the event of a bioterrorist attack, one of the major power's fears.

Mike Hammer, a spokesman for the US National Security Council, announced the decision to hang on to stocks.

"There has been a growing concern about the threat of bioterrorism," he said.

"There are elements out there that might have some stock that they could use in a terrorist attack."

He said the US did not want to take any chances.

"It is nearly impossible to be 100% sure that the disease has been completely eradicated, and we believe that the prudent course is to develop vaccines so that we can cope with this in the case of an inadvertent, or a deliberate, attempt to spread the disease."

Samples may have escaped government control

Information from former Soviet research official Ken Alibek, who defected to the United States in 1992, suggests the virus has been distributed to places in Russia beyond the known laboratories - possibly to places with less effective security controls.

Specialists at a recent Johns Hopkins University conference on bioterrorism said that Mr Alibek's information has been verified by other sources.

Mr Hammer said the US would call for a delay in the destruction of smallpox stocks at a meeting of the World Health Assembly, the WHO's governing body, next month.