WASHINGTON — The U.S. intelligence community has disputed an assessment by its Israeli counterpart and the Pentagon regarding Iran's nuclear program. In an assessment that played down warnings by U.S. military chiefs, the American intelligence community determined that Iran has failed to produce highly-enriched uranium, required for the assembly of nuclear weapons. Two leading U.S. intelligence officials said Iran was not known to have begun refining low-enriched uranium into fuel that could be installed into a missile warhead.