UNITED NATIONS, N.Y. — Under pressure from the United Nations Security Council, Iraq has provided a long list of nuclear-related materials which United States officials say prove that Iraq has been running a clandestine atomic weapons program.In a letter to the UN July 8, Iraq admitted to having uranium-enrichment programs that it had not previously declared to the International Atomic Energy Agency (IAEA). US and British diplomats familiar with Iraq's 29-page disclosure said Baghdad contended the experiments were for peaceful purposes and not for a weapons program. But the diplomats were skeptical. "It is quite impossible for a country that has violated IAEA safeguards and has unsupervised programs to credibly claim they were doing something peaceful," a US State Department official said. In Washington, White House spokesman Marlin Fitzwater welcomed the Iraqi move, but added that "it appears that nothing in this document changes the need for comprehensive inspections." The disclosure came more than a week after Iraqi forces had thwarted a UN team's attempts to inspect several sites near Baghdad. The IAEA, in a statement, said the letter showed Iraq had three parallel programs for uranium enrichment and related equipment and facilities - described as centrifugal, electromagnetic, and chemical separation processes. Iraq has regularly declared it possessed uranium for peaceful uses to the IAEA and placed it under safeguards. In early May, it disclosed that it had 80 pounds of enriched uranium. Under an April 3 Security Council cease-fire resolution Iraq must yield its nuclear materials, chemical and biological weapons, and ballistic missiles to the UN for destruction.