The International Atomic Energy Agency said its planned visit to Iran next week has been postponed at the request of Tehran. Western diplomats say the delay raises questions about Tehran's credibility.
Two senior IAEA experts were to leave for Tehran Sunday, and a follow-up team was to join them next week.
But the Iranians said they want more time to prepare for the inspections, and have asked the IAEA for at least another week. Iran has until the end of October to answer all the agency's questions on its nuclear program, which it claims is purely peaceful.
The IAEA said its scientists want to discuss inconsistencies in Iran's account of tests on gas centrifuges that could be used for a nuclear weapons program.
The delay follows media reports that international inspectors have found more traces of highly enriched uranium at locations in Iran.
Western diplomats say Iran could provide all the information the agency is seeking relatively quickly, and see the delay as adding to an already big "confidence deficit."
IAEA spokesman Mark Gwozdecky said this is not the first time Tehran has delayed a visit from the agency. "A year ago, almost exactly, we had the first revelations of these new facilities in Iran, and a year later, we're still proceeding in a very unsatisfactory manner. First, they brought us there in February - after a long delay, they finally allowed [IAEA chief Mohamed] ElBaradei to go in February," he said.
The IAEA is hoping this new delay will still give its experts enough time to analyze material, and make a report for the next Board of Governors meeting at the end of November.
Iran has just lost its seat on the board, due to a normal rotation in membership. So, it will have no vote, if the IAEA decides to refer its nuclear program to the U.N. Security Council.