The United States has been secretly seeding clouds over North Vietnam, Laos and South Vietnam to increase and control the rainfall for military purposes. Government sources, both civilian and military, said during an extensive series of interviews that the Air Force cloud seeding program has been aimed most recently at hindering movement of North Vietnamese troops and equipment and suppressing enemy antiaircraft missile fire. The disclosure confirmed growing speculation in Congressional and scientific circles about the use of weather modification in Southeast Asia. Despite years of experiments with rainmaking in the United States and elsewhere, scientists are not sure they understand its long term effect on the ecology of a region. The weather manipulation in Indochina, which was first tried in South Vietnam in 1963, is the first confirmed use of meteorological warfare. Although it is not prohibited by any international conventions on warfare, artificial rainmaking has been strenuously opposed by some State Department officials. According to interviews, the Central Intelligence Agency initiated the use of cloud seeding over Hue, in the northern part of South Vietnam. "We first used that stuff in about August of 1963," one former C.I.A. agent said. "The agency got an Air America Beechcraft and had it rigged up with silver iodide. There was another demonstration and we seeded the area. It rained." A similar cloud seeding was carried out by C.I.A. aircraft in Saigon at least once during the summer of 1964.