The International Atomic Energy Agency is sending expert teams to Iran for a month to inspect Tehran's nuclear facilities and try to find answers to some unresolved questions.
A team of senior IAEA inspectors will leave for Tehran this weekend, and another group of experts will fly out next week. Both are expected to work throughout October, inspecting Iran's nuclear facilities and trying to clear up inconsistencies in Tehran's own reports to the nuclear agency.
The U.N. nuclear watchdog has given Iran until the end of October to prove it does not have a nuclear weapons program.
Of particular concern to the IAEC are gas centrifuges that Iran claims it developed in university laboratories without testing live nuclear material. IAEC spokesman Mark Gwozdecky says the agency's scientists do not find Tehran's explanations credible.
"Our experts went there in June and concluded that this was not plausible and that's in the report we issued in August," he said. "That our experts were basically saying we don't believe you could have built a car and not put some petrol in it to make it run properly."
Centrifuges are used to purify natural uranium so that it can be used as nuclear fuel for reactors or for weapons.
IAEC would not confirm media reports that its inspectors found more weapons grade uranium in Iran at a non-nuclear location. Iran delayed inspections to the site in the summer, and scientists taking samples said they saw evidence of significant modifications.
Even if highly enriched uranium has been found, IAEC says, more tests would be necessary to find out its origins.
Iran is currently not obligated to give international inspectors access to its non-nuclear facilities. But the head of the IAEA, Mohamed ElBaradei, has told a radio interviewer Iran's failure to cooperate could have serious consequences.