The UN nuclear agency says Iran has rapidly ramped up production of higher-grade enriched uranium over the last four months.
The statement was made in a confidential report that feeds concerns about how quickly the Islamic Republic could produce an atomic bomb.
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Friday's report by the International Atomic Energy Agency also said Iran had failed to give a convincing explanation about a quantity of missing uranium metal.
Diplomats have said the missing amount could be used for experiments used to arm a warhead.
Iran insists it is not interested in nuclear weapons and that all of its activities are meant either to generate energy or to be used for research.
But the IAEA expresses "serious concerns regarding possible military dimensions to Iran's nuclear program."
The UN's atomic agency said it has "major differences" with Iran and "major concerns" about its nuclear program.
"An intensive discussion was held on the structured approach to the clarification of all outstanding issues related to Iran's nuclear program. No agreement was reached between Iran and the Agency, as major differences existed with respect to this approach," the IAEA said following its latest failed mission to Tehran.
The IAEA said that Iran has tripled production of 20% enriched uranium since its last assessment in November, with 696 centrifuges installed at its heavily bunkered Fordo site.
Uranium further enriched to 90% could in theory be used in a nuclear bomb, although Iran denies intending to do so, saying its activities are peaceful.
Iranian officials repeated their assertion during the recent visits that the report, which has prompted Western countries to ramp up sanctions, was "based on forgeries."
In Washington, State Department spokesman Mark Toner declined to comment on the details of the report, which he said officials were still studying, but added it contained "nothing that has allayed our concerns about Iran’s nuclear program."
White House National Security Council spokesman Thomas Vietor said Iran's continued uranium enrichment, "combined with its continued stonewalling of international inspectors... demonstrate why Iran has failed to convince the international community that its nuclear program is peaceful."
AP and AFP contributed to this report
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