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 Usama bin Laden's interest in the disease, the U.S. official said, speaking on condition of anonymity.
White House spokesman Ari 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.
Smallpox was declared eradicated worldwide in 1980, but concerns that it may be revived for use as a weapon have prompted the Bush administration to consider vaccinating U.S. citizens and to prepare emergency plans should an outbreak be detected.
The disease historically has killed about a third of its victims and can be transmitted from person to person, unlike some other biological weapons such as anthrax.
Smallpox has plagued mankind for centuries, and is believed to have killed more people than all wars and epidemics combined. Death typically is caused by massive hemorrhaging.
France is thought to have small amounts of the virus for use in programs aimed at researching and mounting a defense to an outbreak of the disease, U.S. officials said.
Russia is thought to have a great deal more smallpox in its stockpiles than the small amount allowed for them under international agreements, according to the officials.
Ken Alibek, a former top scientist in the Soviet biological weapons program who came to the United States in 1992, said the Soviets covertly developed smallpox as a weapon in the 1980s.
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, they said.
U.S. officials believe Iraq and North Korea have chemical weapons and biological weapons other than smallpox.
Over the years, there have been reports that Libya, Syria and Iran also have smallpox samples.