Iran temporarily halted lower-level uranium enrichment work earlier this month, a UN nuclear watchdog report said, after Western diplomats said Tehran's nuclear program was suffering technical problems.
The confidential report, obtained by Reuters on Tuesday, did not give any reason for the unusual move by Iran to briefly stop feeding material into centrifuge machines used to refine uranium.
It also said the International Atomic Energy Agency (IAEA) remained concerned about possible activity in Iran to develop a nuclear payload for a missile.
Despite the temporary halt of enrichment work in mid-November, Iran's total output of low-enriched uranium (LEU) rose to reach 3.18 tons, 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 much further.
A diplomat close to the IAEA said none of the centrifuge units, or cascades, at Iran's Natanz plant were being fed for enrichment to lower-levels when inspectors visited the site on Nov 16.
"They weren't enriching uranium," he said.
About a week later, Iran informed the UN agency that 28 cascades - each normally containing 164 centrifuges - were enriching uranium again.
The diplomat said he did not know why Iran had temporarily stopped the work or for how long it had lasted. But he suggested a technical issue was the likely reason. He said it had happened a few times in the past, without giving details.
Western diplomats earlier on Tuesday said it was unclear whether the Stuxnet computer virus may have been to blame for technical problems they said Iran had been experiencing.
Iran is using an old centrifuge model which has been dogged by previous breakdowns.
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 program.