International nuclear inspectors reported on Friday that Iran had dramatically increased its capability to produce nuclear fuel over the summer, even while slowing the pace at which it was enriching the uranium that the West fears could one day fuel nuclear weapons.
The slowdown puzzled the inspectors, and Iran offered no clues about whether technical problems or political considerations accounted for its action.
Nonetheless, outside nuclear experts who dissected the agency's latest report — a crucial one because it comes just as the United States and its European allies are debating far more damaging sanctions — said that if Iran's current stockpile of low-enriched uranium were further purified, it would have nearly two warheads' worth of bomb fuel.
A senior Iranian envoy angrily denounced the assessment as "fabrication," insisting his country has gone out of its way to be transparent and cooperative. Iran insists its nuclear program is peaceful and geared solely toward generating electricity. The United States and important allies contend the country is covertly trying to build an atomic weapon.
In its report on Friday, the International Atomic Energy Agency also said Iran has reopened some key sites to inspectors, after keeping them out for a year. But the agency said that after years of requests, the country still refuses to turn over important documents linked to suspicions its military has been involved in the nuclear program, or to allow the agency to interview key personnel who are suspected of roles in weapons development.
The slowdown in the enrichment of uranium — the key ingredient Iran would need to produce to fuel nuclear power plants or nuclear weapons — was something of a surprise. For reasons the agency could not explain, Iran has reduced, at least for now, the number of centrifuges it is actively using for enrichment, from 4,920 in June to 4,592 — a decrease of 328 machines.
The new report also disclosed that Iran had increased the number of centrifuges that were installed and ready to add to Iran's capacity. It put the new total at more than 8,300 — or an increase of more than 1,100 centrifuges since June.
"Continuing to install large numbers of new centrifuges is significant" since filling them with raw uranium "is a relatively minor step," the Institute for Science and International Security, a private group in Washington, said in an analysis of the new report.
Information from the Associated Press was used in this report.