Washington has dismissed a United Nations report that finds no evidence Iran has been seeking atomic weapons.
John Bolton, the top US arms control official, said the International Atomic Energy Agency assessment was "impossible to believe".
He said Iranian efforts to acquire nuclear capabilities only made sense as part of a weapons programme.
The IAEA report leaked this week said Iran had admitted to producing high-grade plutonium, but not weapons.
It said there was no sign that Tehran had secretly been developing weapons but had nonetheless admonished the Iranian authorities for being secretive.
"To date there is no evidence that [Iran's] previously undeclared nuclear material and activities... were related to a nuclear weapons programme," the agency said.
"However," it added, "given Iran's past pattern of concealment, it will take some time before the agency is able to conclude that Iran's nuclear programme is exclusively for peaceful purposes."
Iran has always claimed its nuclear programme is designed to meet the country's energy needs.
However, Mr Bolton, speaking at a dinner for the US publication The American Spectator, threw scorn on Tehran and the IAEA.
"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 programme," he said.
But he stopped short of directly criticising the head of the IAEA, Mohammed ElBaradei, according to BBC correspondent in Washington David Bamford.
The tone of Mr Bolton's statement, our correspondent adds, indicates the US is prepared to take on the UN's nuclear body and state contrary conclusions.
Mr Bolton had wanted to confront Iran in the UN Security Council, while others, including the Europeans, have sought quiet diplomacy to resolve the issue.