Iran has started moving nuclear material to an underground facility for the pursuit of sensitive atomic activities, a UN nuclear agency report said, a development likely to add to Western suspicions Tehran is trying to build a weapon.
The International Atomic Energy Agency document also said Iran had continued to stockpile low-enriched uranium (LEU) and one prominent US think-tank said it had enough of the material for four nuclear weapons if it refines it further.
The information that Iran last month moved a "large cylinder" with LEU to the Fordow subterranean site was included in the UN body's most comprehensive report yet pointing to military aspects of Tehran's nuclear programme.
The main finding in the IAEA report, which was leaked on Tuesday, was that Iran appeared to have worked on designing a nuclear warhead and that secret weapons-relevant research may continue.
It may pave the way for further Western sanctions on the major oil producer. France warned on Wednesday that Tehran faced unprecedented punitive measures if it refused to cooperate with the IAEA over its nuclear programme
"No unbiased observer can cling to the pretension that Iran's nuclear programme is purely for peaceful purposes," Mark Fitzpatrick, a director of the International Institute for Strategic Studies think tank, said.
But he said Iran was not yet actually building atomic bombs and it was still more than a year away from being able to do so, should its leaders decide to.
The IAEA report also contained updated information about Iran's uranium enrichment -- the part of Iran's nuclear work that has most worried the West as refined uranium can be used for arms if processed further.
Iran's decision in early 2010 to raise the level of some enrichment from the 3.5 per cent purity needed for normal power plant fuel to 20 per cent worried Western states that saw this as bringing it closer to the 90 per cent needed for a bomb.
Iran's main enrichment plant is located near the central town of Natanz. But the country announced in June it would move its higher-grade activity to Fordow, offering better protection against any military attacks.
This week's IAEA document showed that Iran had now installed two sets of 174 machines each for refining uranium to a fissile purity of 20 per cent at Fordow near the holy city of Qom. The centrifuge machines were not yet operational.
In its previous report on Iran's nuclear programme, in early September, the IAEA said Iran had installed one of two planned cascades, or interlinked networks of centrifuges. Such machines spin at supersonic speeds to increase the fissile isotope ratio.
On 17 October, the latest report said, Iran "transferred ... one large cylinder containing LEU in the form of UF6 (uranium hexaflouride gas) and one small cylinder containing depleted uranium in the form of UF6."
The uranium gas is fed into centrifuge machines to refine uranium for use either to fuel nuclear plants, Iran's stated aim, or provide material for the core of a nuclear bomb, which the West fears is Iran's ultimate goal.
"According to Iran, the LEU will be used for feeding and the DU will be used for line passivation," the IAEA report said, referring to technical preparations for starting enrichment.
Iran only disclosed the existence of Fordow -- tucked deep inside a mountain on a former military base -- to the IAEA in September 2009 after learning that Western intelligence agencies had detected it.
Tehran says it will use 20 per cent-enriched uranium to convert into fuel for a research reactor making isotopes to treat cancer patients, but Western officials say they doubt that the country has the technical capability to do that.
Western experts say tightening sanctions, technical hurdles and possible cyber sabotage have slowed Iran's atomic advances.
But it is still amassing LEU: the IAEA report said it had produced a total of more than 4.9 tonnes since the work started in February 2007, some of which has been converted into 20 per cent material.
"This amount of low enriched uranium if further enriched to weapon grade is enough to make four nuclear weapons," said the Institute for Science and International Security, a Washington-based think tank.