The head of the UN nuclear watchdog has rejected criticism for its failure to detect Iran's clandestine experiments to make enriched uranium and plutonium.
In an interview with reporters, Dr Mohamed ElBaradei, Director-General of the UN International Atomic Energy Agency (IAEA), also said Iran had yet to sign a protocol accepting more intrusive snap inspections, though diplomats said it was too early to say whether Iran was stalling.
In October, Iran acknowledged to the IAEA that it hid a secret centrifuge uranium enrichment programme from UN inspectors for nearly two decades.
Dr ElBaradei said Iran's laboratory-scale experiments, which Washington said were further proof that Tehran has been secretly trying to develop nuclear weapons, were on too small a scale to be easily detected by IAEA nuclear inspectors.
"People have been saying Iran has been cheating the agency, if you like, for 18 years," Dr ElBaradei said. "Yes, Iran has been successful in doing research and laboratory activities and this we were not able to detect, and I don't think we will be able to detect in the future."
"But...if a country moves from research...to an industrial scale to develop weapons, I think the system, with all the technology that we have, makes it highly unlikely that this kind of programme would go on undetected," Dr ElBaradei said.
The United States accuses Iran of using its nuclear power programme as a front to build an atomic bomb, a charge Tehran denies.
While the IAEA concluded in a recent report that it had seen "no evidence" Iran did have a covert weapons programme, it said the jury was still out as to whether one existed.