The findings may change the calculus of diplomacy in Europe and in Washington, which has aimed to force a suspension of Iran's enrichment activities in large part to prevent it from learning how to produce weapons-grade material.
In a short-notice inspection of Iran's main nuclear facility at Natanz on Sunday, conducted in advance of a report to the United Nations Security Council due early next week, the inspectors found that Iranian engineers were already using roughly 1,300 centrifuges and were producing fuel suitable for nuclear reactors, according to diplomats and nuclear experts here. Until recently, the Iranians were having difficulty keeping the delicate centrifuges spinning at the tremendous speeds necessary to make nuclear fuel, and often were running them empty, or not at all.
Now, those roadblocks appear to have been surmounted.
"We believe they pretty much have the knowledge about how to enrich,"said Mohamed ElBaradei, the director general of the energy agency, who clashed with the Bush administration four years ago when he declared that there was no evidence that Iraq had resumed its nuclear program.
"From now on, it is simply a question of perfecting that knowledge. People will not like to hear it, but that's a fact."
It is unclear whether Iran can sustain its recent progress. Major setbacks are common in uranium enrichment, and experts say it is entirely possible that miscalculation, equipment failures or sabotage could prevent the Iranian government from reaching its goal of producing fuel on what President Mahmoud Ahmadinejad boasts is "an industrial scale."