Iran's nuclear program has suffered a recent setback, with major technical problems forcing the temporary shutdown of thousands of centrifuges enriching uranium, diplomats said on Monday.

The diplomats said they had no specifics on the nature of the problem that in recent months led Iranian experts to briefly power down the machines they use for enrichment - a nuclear technology that has both civilian and military uses.

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 concern because it can be re-engineered to produce uranium for nuclear warheads.

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 choose 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 UN 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 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.