GENEVA — During the height of the Cold War, the Soviet Union armed strategic ballistic missiles with the deadly smallpox virus and readied them for use against the United States, a former Soviet military biologist said here yesterday.

The Soviet Union had the missiles armed and ready to launch “in the beginning of the 60s,” according to Ken Alibek, a former senior scientist in Moscow’s biological warfare research program. Alibek defected to the United States in 1992 and is now the executive director of George Mason University’s Center for Biodefense in Virginia.

Alibek spoke at the Smallpox Biosecurity conference here, which was sponsored by smallpox vaccine producer Acambis.

He said the smallpox missiles were classified as strategic weapons, which Soviet officials never intended to use except in the most dire circumstances.

Alibek also said the smallpox virus was extremely potent and was thoroughly tested by Soviet researchers.

It was “absolutely obvious” that the effect of some biological weapons “would exceed some forms of nuclear weapons,” he said.

The little-known Soviet program sought to defeat the technical problems of preserving biological agents during the re-entry phase of the missile’s flight and of dispersing the agents over a wide area.

“It has always been considered extremely difficult to deploy or deliver biological weapons in a militarily significant way,” said Jon Wolfsthal, deputy director of the Carnegie Endowment for International Peace’s Nonproliferation Project.

“If Alibek is right, this would be the only known case of strategically deployed biological weapons,” he said.

Over the past four decades, there have been reports of Soviet research into techniques of protecting and dispersing biological agents on missiles, according to John Pike of GlobalSecurity.org. The reports indicated research into warhead cooling systems to preserve the agent, warhead parachutes to slow their re-entry and the dispersal of the biological agent through smaller bomblets contained in the warhead.

“They had conducted tests at the time that involved decelerating the warhead to the point that it could disperse the biological agent,” Pike said.

“The Soviets did a lot weird stuff. They were pretty frisky,” he added.