The disclosure of the unaccounted third came in the atomic agency’s quarterly report to its board, which was made public on Thursday. The report noted that Iran had now produced a total of 1,010 kilograms  or 2,227 pounds  of low-enriched uranium.

The discrepancy came to light when the report noted that the new total came from the addition of 171 kilograms of new production to 839 kilograms of old production. But the agency had previously reported the old production as 630 kilograms.

So the Iranians had actually made far more uranium than previously disclosed  209 kilograms more, an increase of a third. That amounts to a little more than 460 pounds.

The United Nations’ officials explained the discrepancy as resulting from Iran’s estimates versus careful measurement. They called the inconsistency reasonable for a new enrichment plant.

The officials dismissed suggestions that the discrepancy meant that Iran could smuggle enriched uranium out of the Natanz plant for processing at a secret location. “We’re sure that no material could have left the facility without us knowing,” the senior United Nations official said. But he admitted that the inspection teams do their own inventory just once a year. “It’s only at that moment,” he said, “that we have our own independent data.”

The report also gave updated figures for Iran’s use of centrifuges  the machines that spin incredibly fast to enrich uranium into nuclear fuel. At Natanz, it said, Iran is feeding uranium into about 4,000 centrifuges and has 1,600 more in the wings, for a total of 5,600. That compares with 3,800 working centrifuges listed in the agency’s November report.

In Paris earlier this week, the head of the United Nations nuclear agency, Mohammad ElBaradei, said Iran appeared to have made “a political decision” to do less enrichment than it physically could. The Security Council has imposed sanctions on Iran for failing to suspend enrichment, which can be used to make fuel for nuclear reactors or bombs. While Iran insists that its efforts are entirely peaceful, the United States and other Western nations see the enrichment as a bid for atom bombs.