|Nuclear inspectors witnessed the stoppage at the Natanz enrichment plant in central Iran [EPA]
Iran temporarily halted some lower-level uranium enrichment a week ago after apparently discovering a glitch in the system, according to a leaked report by the International Atomic Energy Agency (IAEA).
Nuclear inspectors monitoring Iran witnessed the shutdown at the Natanz enrichment plant in central Iran on November 16, the confidential report said.
It was unclear whether the shutdown lasted just hours, days, or longer as the inspectors were only at the site for about two hours.
The UN nuclear watchdog suggested no reason for the shutdown, which saw Iran briefly stop feeding material into centrifuge machines used to refine uranium.
Iran is using an old centrifuge model which has been dogged by previous breakdowns.
A senior diplomat familiar with the agency's monitoring of Tehran's atomic activities said the Iranians gave the IAEA inspectors no explanation for the incident.
About a week after the stoppage, Iran informed the UN agency that 28 cascades - each normally containing 164 centrifuges – were enriching uranium again.
Western diplomats said earlier on Tuesday that Iran had been experiencing technical problems at its nuclear sites and it was unclear whether the Stuxnet malicious software virus was to blame.
Over the past several months, Iranian officials have acknowledged that the Stuxnet code had spread widely through Iranian industrial sites and infected several personal laptops belonging to employees at the country's first nuclear power plant.
However, it has denied that it has infected the operating systems at any nuclear site.
"One year and several months ago, Westerners sent a virus to [our] country's nuclear sites," Ali Akbar Salehi, Iran's nuclear chief, said on Tuesday, according to the Islamic Republic News Agency (IRNA), the official Iranian news agency.
"They had hoped to stop our speedy peaceful nuclear activities through software. But, with the grace of God, we discovered the virus exactly at the same spot it wanted to penetrate because of our vigilance and prevented the virus from harming [equipment]."
He did not specify which sites were targeted.
Security experts have said the release of Stuxnet could have been a state-backed attack, possibly from Israel or another foe of Iran, to sabotage the Islamic republic's nuclear programme.
Despite the temporary halt to enrichment work in mid-November, the report concluded that Iran's total output of low-enriched uranium (LEU) rose to reach 3.18 tonnes, suggesting Iran had maintained steady production in recent months.
Experts say that amount could be enough for at least two bombs if refined much further.
The report came ahead of a possible resumption next month of talks between Tehran and major powers over Iran's nuclear programme, which the US and its allies suspect is aimed at making weapons but which Iran says is designed to produce electricity.