Expectations were high just two weeks ago, when chief UN nuclear inspector Yukiya Amano emerged from talks in Tehran with Iranian President Hasan Rouhani saying Iran had given "a firm commitment" of cooperation.
"We have started and that is important," Amano said, suggesting that the years of deadlock had been broken.
His high-profile trip was meant to kick-start the latest effort to investigate the allegations. The investigation was agreed to in February but had made little progress.
IAEA spokeswoman Gill Tudor said today the agency would have no comment. Iran's mission to the IAEA said Reza Najafi, the chief delegate to the agency, was in Tehran and nobody else could talk to reporters.
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The IAEA inquiry is formally separate from those U.S.-led talks. But Washington says that a final deal must include a conclusion by the IAEA that it has satisfactorily completed its investigation.
The Iran-six power talks already have been extended from a July 31 deadline due to wide differences between the two sides.
Iran's reported refusal to advance the probe is bound to embarrass Amano considering his optimistic comments after his August 17 talks with Rouhani.
Iran and the IAEA agreed in February to a new start to the probe after a decade of deadlock, marked by Tehran's insistence that the allegations were based on falsified intelligence from the United States and arch-foe Israel.