Major technical problems have forced thousands of Iran's nuclear centrifuges enriching uranium to temporarily shut down, diplomats said.
The diplomats said they had no details 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 programme, which experts have identified as being calibrated to destroy centrifuges by sending them spinning out of control.
Iran said its enrichment efforts are geared only to make nuclear fuel, but the programme has aroused international concern because it can be re-engineered to produce uranium for nuclear warheads.
There have been hints that the programme is beset by technical problems. Even a brief shutdown of the thousands of enriching machines would be the strongest documentation to date that the programme - Iran's nuclear cornerstone and a source of national pride - is in trouble.
The country has continued to enrich despite increasingly strict United Nations 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 programme.
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.