NEW YORK - In the event of a Cold War nuclear attack, the Soviet Union planned to finish off the United States with a smallpox plague intended to wipe out any survivors, according to a new book and an interview with one of the attack's main planners. "Smallpox biological weapons were intended for use against U.S. cities in a war of total mutual annihilation, with the aim of killing the survivors in the aftermath of a nuclear exchange," writes Jonathan Tucker of the Monterey Institute of International Studies in his new book, "Scourge: The Once and Future Threat of Small Pox." NBC News obtained an advance copy of the book, which will be published by Atlantic Monthly Press next month. The Soviet program's chief scientist at the time has confirmed Tucker's assessment and said that Mikhail Gorbachev, then general secretary of the Communist Party, knew about the program. "Believe me, depending on a specific scenario, of course, we would see very, very large numbers of infected and killed people," Kanatjan Alibekov, known since his 1992 defection as Ken Alibek, told NBC News. U.S. intelligence officials said they were unaware of the plan until Alibek defected but had suspicions that one Soviet missile system had been modified to carry biological weapons. Alibek later wrote about his experiences in his book, "Biohazard." So serious was Soviet planning for such a war, Tucker reports, that warheads on at least four Soviet ICBMs - the SS-11, SS-13, SS-17 and SS-18 - were equipped with special biological weapon warheads over a 20-year-period. Many of the missiles "were based in silos near the Arctic Circle on a launch-ready status," Tucker writes. "The cold temperatures in the far north kept the smallpox agent viable for long periods." FIELDS NOT ACTIVE Tom Cochran of the Natural Resources Defense Council, which maintains a historical database of Soviet missile deployments, said that while there were no Soviet missile fields within 500 miles of the Arctic Circle, four fields of SS-11, SS-13 and SS-17 missiles were located at northern latitudes of the Soviet Union during the period Alibek says the smallpox warheads were deployed. Those fields no longer exist. Tucker says Soviet engineers later developed special refrigerated warheads for the more modern SS-18s "to enable the biological payload to survive the intense heat of re-entry through the atmosphere." A senior U.S. intelligence official at the time confirmed that U.S. spy satellites had detected a variant of the SS-11 missile warhead that had raised suspicions about biological weapons. "We saw that some of the SS-11 missiles had an oddly shaped warhead, but we could not determine precisely what it was for. But at the time we suspected it might be for biological weapons," said the official, who spoke on condition of anonymity. The ultimate effect of such an attack, according to Tucker, would have been nothing short of eliminating American society: "A Soviet attack with smallpox weapons against urban targets would have dealt a devastating blow to the United States, perhaps destroying it as a functioning society." Smallpox claimed hundreds of millions of human lives, far more than any other infectious disease, until it was eliminated in 1978. It can spread easily - through coughing, sneezing or physical contact - and can kill up to 30 percent of those it infects within 15 to 20 days. Small pox peppers the face, trunk, extremities, mouth and throat with pus-filled boils, and even corpses can remain highly contagious. CHINA ALSO TARGETED Alibek, who now lives in northern Virginia, said he and his colleagues were aware that American cities were targeted and that discussions also included "technical" details of which cities would be struck by biological warfare missiles, some of which were to loaded with smallpox, others with anthrax. "You are not supposed to know what country would be a possible target," said Alibek, who was chief scientist and first deputy director of the Soviet biological weapons program. "But it was absolutely clear that for this type of missiles, one of the main targets was the United States. Alibek said the initial targets were New York, Seattle and Chicago, and that Boston was added to the list later. And American cities were not the only target, reports Tucker. After 1968, Chinese cities also were placed on the target list. Alibek said he saw Gorbachev's signature on a Soviet Politburo document authorizing the production of smallpox for use in a war against the United States. Mankind's weapons of terror "Somebody says, 'he didn't know,' " Alibek said. "I knew what I saw, I knew what he knew. ... It was signed in February, in February of 1986. All projects to develop new smallpox biological weapon." Tucker says the Soviet Union may have been responsible for distributing samples of the smallpox virus to other countries, including Iraq and North Korea, following the World Health Organization's eradication of the disease in the late 1970's. Tucker cites a U.S. National Security Council document as listing other possible recipients as China, Cuba, Indian, Iran, Israel, Pakistan and Yugoslavia. The same document, first reported by The New York Times, noted that there was circumstantial evidence that Iraq's research into camel pox was a surrogate for small pox. Robert Windrem is an NBC News investigative producer based in New York. http://www.msnbc.com/news/616710.asp