The United Nations' nuclear watchdog says Iran is pushing ahead with higher-level uranium enrichment and has failed to answer its questions about possible military dimensions to its nuclear programme.
The International Atomic Energy Agency (IAEA) also said Tehran refused to address concerns about possible undisclosed nuclear activities in a report the US said "clearly shows Iran's continued failure to comply with its international obligations and its sustained lack of co-operation with the IAEA".
The report also said Iran had added a second set of 164 centrifuges - nuclear enrichment machines - to help refine the uranium but they were not yet operational. At the time of the previous report in February, Iran had only one set of centrifuges installed for the work, the IAEA said.
Tehran started refining uranium to the higher level of 20 per cent in February, saying it wanted to produce fuel for a medical research reactor after talks on a fuel supply agreement with Western powers stalled.
Last month, Brazil and Turkey resurrected parts of the proposal, under which Iran would ship 1.2 tonnes of its low-enriched uranium stockpile abroad in return for the fuel, seen as a way to reduce tensions with the West.
|Iran has vowed it will continue its nuclear programme despite growing pressure [AFP]
But the new IAEA report said Iran's low-enriched uranium stockpile had grown to 2.4 tonnes, so that even if the 1.2 tonnes was shipped out now, it would still leave Iran enough material for a nuclear weapon if enriched to higher levels.
Iran says its nuclear work is for peaceful uses only.
But major world powers have recently backed a draft UN resolution that would authorise a fourth round of sanctions against its nuclear work.
"Based on this report Washington is going to feel justified in downplaying the Brazilian-Turkish-Iranian deal and focusing on sanctions instead," said David Albright, the head of the Institute for Science and International Security.
Iran's raising of the enrichment level to 20 per cent has heightened Western suspicions because it takes the material closer to the 90 per cent purity needed to make atomic weapons.
The country is also thought to lack the capability to make the special fuel assemblies needed for the medical research reactor.
No swap deal
Meanwhile on Monday, the Iranian ambassador to Iraq said the recent release of two Iranians captured by US forces in Iraq was not an indication of any impending deal to free three Americans held by Tehran on spying charges.
Hassan Kazemi Qomi told the Associated Press news agency that the fate of the Americans, who have been held since July, is in the hands of the Iranian judiciary and has no connection to the release of two Iranians earlier this month.
"There were no deals. They are in the custody of the judiciary system.''
The detained Americans - Sarah Shourd, Shane Bauer and Josh Fattal - were arrested along the Iraqi border and accused of being spies.
Their families say they were hiking in northern Iraq's mountainous Kurdish region and if they strayed into Iran, it was unintentional.