Washington has dismissed a United Nations report that finds no evidence Iran has been seeking atomic weapons.
The IAEA sees no sign of a nuclear weapons programme so far
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.
The Iranian ambassador at the IAEA, Ali Akbar Salehi, said the facts in the report made it "very difficult to refer the issue to the Security Council".
"Every logical person knows that Iran has never intended to pursue the path of weapons of mass destruction," he told the BBC's World Today programme.
He added: "We cannot convince somebody who doesn't want to be convinced."
Earlier the Iranian President, Mohammad Khatami, had given a positive reaction to the report, saying it would prevent certain powers, as he put it, from moving against Iran.
Iran also pledged to suspend its uranium enrichment from Tuesday and said it will allow tougher UN inspections of its nuclear facilities.
The former UN chief weapons inspector in Iraq, Hans Blix,
has said there was no direct evidence Iran to support the claim that Tehran was using a civilian energy programme to make a nuclear bomb.
But Israeli Defence Minister Shaul Mofaz speaking in Washington said Iran's nuclear programme could reach "the point of no return" within a year.
In September, the IAEA Board of Governors gave Iran a 31 October deadline to give details of its nuclear programme.
To meet the deadline, Iran made a number of admissions to hidden activities, according to the report.
The Vienna-based IAEA is to meet later this month to decide whether to declare Iran in violation of the Nuclear Non-Proliferation Treaty.