US intelligence believes that four countries other than the United States possess hidden stocks of the smallpox virus, US officials say.

Russia, France, Iraq and North Korea have samples of smallpox, according to the officials.

The Washington Post reported that the information came from classified US intelligence briefings.

Smallpox

Has existed for 3,000 years Spreads through the air Fatal in up to 30% of cases No known cure Vaccination before exposure or 2-3 days after offers almost complete protection Vaccination 4-5 days after exposure may prevent death US stopped routine vaccination in 1972 Adults vaccinated as children may no longer be protected Vaccine can have serious side effects Sources: CDC and WHO

The officials also said Osama Bin Laden's al-Qaeda network had tried to obtain the virus and other biological weapons, but that it was unlikely it had the capability to mount an attack with smallpox.

The two assessments are thought to have contributed to US plans for mass vaccination against the disease, which kills up to 30% of its victims.

Iraq and North Korea have both been named by US President George W Bush as part of an "axis of evil", and there are fears that both could develop powerful biological weapons using these samples.

There are also concerns that poor security in Russia could allow other nations or terrorist organisations to obtain smallpox.

Undeclared samples

Smallpox seed cultures are currently officially maintained in just two laboratories - in Atlanta in the US state of Georgia and Koltsovo, Siberia.

The US renounced germ warfare in 1969 has not developed any known offensive programme since.

But the CIA's Weapons Intelligence, Non-proliferation and Arms Control Center now believes that four nations, including Russia, have undeclared samples of the virus.

• Russia: Ken Alibek, a senior scientist in the Soviet biological weapons programme who defected to the US, says the USSR covertly developed smallpox as a weapon in the 1980s. Russia denies this, but Dr Alibek's account is generally accepted by US officials

• France: Stocks are said to be purely defensive, and aimed at preventing casualties in an outbreak of the disease

• Iraq: Evidence of stocks are said to come from UN weapons inspectors, prisoners from the Gulf War found to be immune to the disease and testimony from a Soviet scientist of technology transfer in the early 1990s

• North Korea: Evidence is based on US and Russian intelligence reports on applied military and biological research but is of "medium" rather than "high" quality.

Vaccine stockpile

In July, the US announced a plan to vaccinate 500,000 emergency and health workers.

The vaccine has been produced rapidly and stockpiled since the 11 September attacks on New York and Washington last year.

There are also fears that an attack on Iraq could lead to retaliation using biological weapons.