John Bolton, under secretary of state for arms control and international security, said the International Atomic Energy Agency's conclusion flew in the face of established facts. He stopped short of directly criticizing IAEA director general Mohammed ElBaradei who authored the report.
"After extensive documentation of Iran's denials and deceptions over an 18-year period and a long litany of serious violations of Iran's commitments to the IAEA, the report nonetheless concluded that 'no evidence' had been found of an Iranian nuclear weapons program," he said.
"I must say that the report's assertion is simply impossible to believe," Bolton said in a text released by the State Department prior to its formal delivery as a speech later Wednesday.
Bolton is a hawk whose outspoken views about weapons programs in Iran, Syria and Cuba have caused controversy in the past. However, a State Department official said his remarks on the IAEA report had been approved by top US officials.
The comments are sure to intensify an emerging transatlantic row over how best to deal with Iran. A split became apparent earlier Wednesday when Britain said the report should be dealt with calmly and diplomatically.
The report, released on Monday, accused Iran of conducting two decades of covert nuclear activities, including making plutonium, but said there was no evidence as yet it was trying to build an atomic bomb.
But Bolton was insistent that Iran is in fact attempting to develop a nuclear arsenal under the guise of a civilian atomic energy program.
"In what can only be an attempt to build a capacity to develop nuclear materials for nuclear weapons, Iran has enriched uranium with both centrifuges and lasers and produced and reprocessed plutonium," he said, citing the report.
Bolton charged Iran with trying to cover up the program by repeatedly lying to the IAEA, which oversees the nuclear Non-Proliferation Treaty (NPT) and omitting information from its reports to the agency.
"The United States believes that the massive and covert Iranian effort to acquire sensitive nuclear capabilities make sense only as part of a nuclear weapons program," he said.