The United States has questioned the UN's nuclear watchdog over the latter's claim that Iran has not been engaged in a nuclear weapons programme.
In the first US reaction to a confidential report by the International Atomic Energy Agency (IAEA), US Undersecretary of State Mr John Bolton said its finding that there was no evidence of a nuclear weapons programme was "impossible to believe."
Mr Bolton claimed the report circulated earlier this week reaffirmed the US contention that "the massive and covert Iranian effort to acquire sensitive nuclear capabilities make sense only as part of a nuclear weapons program."
The IAEA report said Iran admitted producing weapons-grade plutonium but that there was no sign Tehran had secretly been developing weapons. President Mohammad Khatami insisted its nuclear policy was purely peaceful.
The former UN chief weapons inspector in Iraq, Mr Hans Blix, said there was no direct evidence Iran was engaged in a civilian energy programme to make a nuclear bomb.
But Israeli Defense Minister Shaul Mofaz said in an address in Washington Iran's nuclear programme could reach "the point of no return" within a year unless there was strong international pressure to stop it.
The IAEA report criticized Iran's nuclear policy, but said there was no proof it had a weapons programme. The agency said it needed more time to establish whether its research was for peaceful purposes, as Iran said it was.
The report said Iran had had a centrifuge uranium enrichment program for 18 years and a high-tech laser enrichment program for 12 years, both of which it had hidden from the United Nations.