Iran temporarily halted most of its uranium enrichment work earlier this month, a UN nuclear watchdog report said, an unusual move which western diplomats said they believed was due to technical problems.
The confidential report, obtained by Reuters news agency, did not say why Iran briefly stopped feeding material into all centrifuge machines used to refine uranium to a low level.
It said the International Atomic Energy Agency (IAEA) remained concerned about possible activity in Iran to develop a nuclear payload for a missile.
Security experts have said the release of the Stuxnet computer virus could have been a state-backed attack, possibly by Israel or another enemy of Iran, aimed at sabotaging the Islamic Republic's nuclear programme.
Any delays in Iran's enrichment campaign could buy more time for efforts to find a diplomatic solution to its stand-off with six world powers - the United States, China, Russia, France, Germany and Britain - over the nature of Tehran's nuclear work.
Talks are due to resume next month between Tehran and the six powers over Iran's nuclear programme, which the West suspects is aimed at making weapons but which Iran says is designed to produce electricity.
Despite the temporary halt in enrichment in mid-November, Iran's total output of low-enriched uranium (LEU) has reached 3.18 tonnes, the report said, suggesting Iran had maintained steady production in recent months.
Experts say that amount could be enough for at least two bombs if refined to a much higher level.
The report also said that despite the shutdown in the middle of the month, Iran was now operating more centrifuge units, or cascades, than it did in August.
Centrifuges are finely calibrated cylindrical devices that spin at supersonic speed to increase the fissile element in uranium so that it can serve as fuel for nuclear power plants or, if refined to a much higher degree, for atomic bombs.
Iran's P-1 centrifuges, adapted from a smuggled 1970s European design which is prone to overheating and vibration, have been plagued by breakdowns since Iran rapidly expanded its enrichment programme in 2007-08.
A diplomat close to the IAEA said none of the cascades, which normally comprise 164 centrifuges, at Iran's Natanz plant were being fed for low-level enrichment when inspectors visited the site on November 16th.
"They weren't enriching uranium," he said.
About a week later, Iran informed the UN agency that 28 cascades were enriching uranium again. Higher-grade enrichment activity continued during the period covered by the report.
The diplomat said he did not know why Iran had temporarily stopped the LEU work or how long the halt had lasted, but suggested a technical problem was the likely reason. He said it had happened a few times in the past, but gave no details.
The head of Iran's Atomic Energy Organisation said Iran's enrichment work had not stopped and that its enemies had not achieved their aims with Stuxnet, which he suggested Tehran had discovered some 18 months ago.
Iran has previously confirmed the virus had infected staff computers at its long-delayed Bushehr nuclear power plant but had not affected major systems there.
"Fortunately the nuclear Stuxnet virus has faced a dead end," ISNA news agency quoted Iranian nuclear programme chief Ali Akbar Salehi as saying.
Iran has tentatively agreed to meet a representative of the six powers, for the first time in over a year, and Salehi said it would announce news about its nuclear programme after the talks which he expected to take place in about two weeks.
Analysts do not expect any immediate breakthroughs in the long-running dispute, which has the potential to set off an arms race in the Middle East and spark a military conflict.
Oliver Thraenert, a senior fellow at the German Institute for International and Security Affairs, said Iran's technical difficulties may have widened the "window of opportunity" for dealing with the issue diplomatically.
Reuters