The Bush administration expressed concern Tuesday that several countries may retain the smallpox virus in violation of international rules.
The comment by State Department spokesman Richard Boucher followed the disclosure by a U.S. official that Iraq, North Korea, Russia and France probably possess hidden supplies of the deadly virus.
Al Qaeda is also believed to have sought samples of smallpox for weaponization, but U.S. officials don't believe the terror network is capable of mounting an attack with smallpox.
Evidence recovered in Afghanistan pointed to Osama bin Laden's interest in the disease, the U.S. official said, speaking on condition of anonymity.
"The general issue of smallpox does remain a concern," White House spokesman Ari Fleischer told reporters Tuesday aboard Air Force One. Fleischer said the administration does not think it likely that al Qaeda has smallpox reserves. The administration is uncertain about Iraq, he said.
Boucher noted that World Health Organization resolutions specify that smallpox virus stocks should be restricted to either the Centers for Disease Control and Prevention in Atlanta or in the Russian city of Vector.
U.S. officials worry that Iraq and North Korea could develop potent biological weapons with their samples, which are believed to exist in small amounts. There is no evidence they can use the disease as a biological weapon. Officials also fear lax security in Russia could allow other nations to obtain the disease for use as a weapon.
The fears that smallpox, declared eradicated in 1980, could again be loose on the world have driven the Bush administration to consider vaccinations for the American populace and to prepare emergency plans should an outbreak be detected.
Smallpox historically has killed about a third of its victims and can be transmitted from person to person, unlike other biological weapons such as anthrax.
Many experts suspected North Korea had samples of the smallpox virus. A Russian intelligence report made public in 1993 accused Pyongyang of having a smallpox weapon, though that has not been publicly corroborated.
A declassified U.S. Defense Intelligence Agency report from May 1994 also quotes an unnamed source saying Russian scientists gave North Korea smallpox samples.
Before 1998, U.N. weapons inspectors discovered limited evidence of a smallpox program in Iraq. They found a machine labeled "smallpox" and Iraq is experimenting with a related virus that infects camels.
Russia acknowledges having samples of the virus, as does the United States. But Ken Alibek, a former top scientist in the Soviet biological weapons program who came to the United States in 1992, claimed the Soviets covertly developed smallpox as a weapon in the 1980s.
U.S. intelligence believes Russia maintains actual stockpiles of the disease, beyond its publicly acknowledged sample in Novosibirsk, the U.S. official said. The U.S. sample is at a government lab in Atlanta.
France's samples are believed to be for defensive research programs aimed at limiting casualties from a smallpox outbreak, the official said.
The Washington Post first reported the intelligence finding in its Tuesday editions.
Routine smallpox vaccinations ended in the United States in 1972, and experts believe that those last vaccinated more than three decades ago have little residual immunity remaining. Only Russia and the United States overtly kept samples of the virus.
But the decision to offer the vaccine is a difficult one because the vaccine itself can be dangerous. It is made with a live virus called vaccinia that can cause serious damage both to people vaccinated and to those with whom they come into close contact.