The United Nations said on Wednesday Iran planned to convert a large amount of raw "yellowcake" uranium into uranium hexafluoride, which one nuclear expert said would be enough to build five atomic bombs.
The International Atomic Energy Agency (IAEA) said in the confidential report circulated to diplomats and obtained by Reuters that Iran planned a "larger test" of a uranium conversion facility "involving 37 tonnes of yellowcake".
David Albright, a former UN weapons inspector and currently president of the Institute for Science and International Security (ISIS) said this could theoretically result in 100 kg of weapons-grade highly-enriched uranium.
Speaking purely hypothetically, Albright said: "It's roughly enough for about five crude nuclear weapons of the type Iran could conceivably build."
The UN agency said it had made much progress in understanding Iran's nuclear programme, though its investigation is not yet complete.
"It is a work in progress," a senior Western diplomat said about the IAEA investigation, which has neither confirmed nor disproved US allegations that Tehran has a secret nuclear weapons programme.
The agency praised Iran for providing it with access to sites inside the country and information, but chided it for being late with the provision of some information.
"In (some) cases, sufficiently detailed information has been so late that it has not been possible to include an assessment of its sufficiency and correctness in this report," the IAEA said.
Washington says Tehran's nuclear programme is a front for a nuclear arms programme. Tehran vehemently denies the charge, saying it is only interested in generating electricity.