Iran is cooperating well with UN nuclear inspectors to help ensure it does not again understate the amount of uranium it has enriched, the International Atomic Energy Agency said on Sunday.
The IAEA statement seemed aimed at quashing any impressions raised by its watchdog report on Iran's disputed nuclear program on Thursday that the accounting shortfall might have been deliberate evasion.
The issue is important due to suspicions, denied by Tehran, that it may put uranium enrichment to making atom bombs and concern about the ability of the IAEA's restricted mission in Iran to keep track of nuclear advances there.
The IAEA report showed a significant increase in Iran's reported stockpile of low-enriched uranium (LEU) since November to 1,010 kg - enough, some physicists say, for possible conversion into high-enriched uranium for one bomb.
'Facility subject to video surveillance'
That figure was based on an amount registered by an IAEA inventory check in November that turned out to be greater by one-third than Iran's own estimate provided to inspectors.
"The (IAEA) has no reason at all to believe that the estimates of LEU produced in the (Natanz) facility were an intentional error by Iran. They are inherent in the early commissioning phases of such a facility when it is not known in advance how it will perform in practice," said IAEA spokeswoman Melissa Fleming.
"Iran has provided good cooperation on this matter and will be working to improve its future estimates," she said.
"No nuclear material could have been removed from the facility without the agency's knowledge since the facility is subject to video surveillance and the nuclear material has been kept under seal."
Fleming said estimates of enrichment output were based on the operator's predictions of how the plant will perform. They are not formal declarations by the country.
The verified LEU figure was based on an inventory check that IAEA inspectors perform once a year.
Such infrequency, some US analysts suggested, meant there was a theoretical risk that any removal of LEU for reprocessing into high-enriched uranium at an undeclared production site might not be swiftly noticed.
David Albright, head of the Institute for Science and International Security in Washington, which closely tracks Iranian nuclear activity, said the IAEA would need to do inventories more often as Iran's LEU stockpile rose.
Iran says it wants a nuclear fuel industry solely to meet rising electricity demand and has promised to keep IAEA monitoring of its two declared nuclear production centers.
But Iran has barred IAEA access elsewhere in reprisal for being hit with UN sanctions for refusing to suspend enrichment and opening up to watchdog inquiries into past activities.
Now off-limits to the IAEA are Iranian research and development plants. Iran is also withholding advance design information about planned facilities such as a heavy-water reactor the West fears could make bomb-grade plutonium. Iran says it will produce isotopes for medicine and agriculture.