Iran has temporarily ceased uranium production in its nuclear facility in Natanz, apparently due to a series of major technical problems.
Diplomats in Vienna said they had no specifics regarding why Iran had shut down production of thousands of centrifuges enriching uranium. But suspicions focused on the Stuxnet worm, the computer virus thought to be aimed at Iran's nuclear program, which experts last week identified as being calibrated to destroy centrifuges by sending them spinning out of control.
Iran says its enrichment efforts are geared only to make nuclear fuel but the program has aroused international concerns because it can be re-engineered to produce uranium for nuclear warheads.
But, there have been hints that the program is beset by technical problems. Even a brief shutdown of the thousands of enriching machines would be the strongest documentation to date that the program - Iran's nuclear cornerstone and a source of national pride - is in trouble.
The country has continued to enrich despite increasingly strict UN sanctions imposed in reaction to its nuclear defiance and has stockpiled enough material for more than two nuclear bombs should it chose to turn it into weapons-grade uranium.
Unease has been fed by Tehran's refusal to accept nuclear fuel from abroad, the covert origins of its enrichment activities and stonewalling of efforts by the International Atomic Energy Agency to probe allegations that it tried to develop components of a nuclear weapons program.
Since being revealed eight years ago, Iran has expanded its enrichment activities to the point where it now runs about 8,500 centrifuges at Natanz in central Iran. But after initial rapid growth, Iranian enrichment capacity has stagnated in recent years. Tehran has taken hundreds of centrifuges off line over the past 18 months, prompting speculation of technical problems.
A U.N official close to the IAEA said a complete stop in Iran's centrifuge operation would be unprecedented to his knowledge but declined to discuss specifics. He, like two like two senior diplomats from IAEA member countries who told the AP of the incident at Natanz, asked for anonymity because the information was confidential.
The three officials spoke on the eve of the planned release of a confidential IAEA update on Iran - the latest report by the Vienna-based agency to its 35-nation board on its attempts to get an overview of Tehran's nuclear activities. The diplomats said it would again focus on Tehran's refusal to heed UN Security Council demands to stop enrichment.
That report will come less than three weeks before planned talks between Iran and the world's five powers - the U.S., Russia, China, Britain, France and Germany - designed to reduce concerns about Tehran's nuclear agenda.
Iran's enrichment program has come under renewed focus with the conclusion of cyber experts and analysts that the Stuxnet worm that infected Iran's nuclear program was designed to abruptly change the rotational speeds of motors such as ones used in centrifuges. Such sudden changes can crash centrifuges and damage them beyond repair.
No one has claimed to be behind Stuxnet, but some analysts have speculated that it originated in Israel.