The UN's nuclear watchdog has rejected sharp US criticism over a report that it has found no evidence Iran is seeking to develop nuclear weapons.
The IAEA sees no sign of a nuclear weapons programme so far
America's top arms control official, John Bolton, said the International Atomic Energy Agency's widely leaked findings were "impossible to believe".
He accused the Islamic Republic of a "massive and covert [nuclear] effort".
Iran has warned of serious consequences if the IAEA refers its nuclear programme to the UN Security Council.
"I hope we do not reach such a stage because then things could very easily get out of control," its envoy to the IAEA, Ali Akbar Salehi, told foreign reporters.
"There are many things Iran can do. We have a lot of leverage," he added without giving details.
The board of governors of the IAEA is due to discuss the report on Iran next Thursday.
The report, which was circulated to officials on Monday, says that Iran has admitted to producing high-grade plutonium but not weapons.
It finds no evidence that Tehran has secretly been developing weapons but nonetheless admonishes the Iranian authorities for being secretive.
"We stand by the report but it's classified and will be considered at next week's board meeting," said IAEA spokesman Mark Gwozdecky.
Iran has always claimed its nuclear programme is designed to meet the country's energy needs.
Mr Bolton, speaking at a dinner for the US publication American Spectator, launched a fierce attack on Tehran and the IAEA's report.
A "massive and covert Iranian effort to acquire sensitive nuclear capabilities", he said, made "sense only as part of a nuclear weapons program".
"The report nonetheless concluded that 'no evidence' had been found of an Iranian nuclear weapons programme... The report's assertion is simply impossible to believe," Mr Bolton said.
The tone of Mr Bolton's statement, BBC Washington correspondent David Bamford says, indicates that the US is prepared to take on the UN's nuclear body and state contrary conclusions.