But in a letter to the UN last month, Tehran admitted to processing a small amount of plutonium in 1995 and also in 1998. The admission came following an analysis by the UN's International Atomic Energy Agency (IAEA) of samples indicating the experiments took place recently.
Shortly after media outlets quoted leaked excerpts from the UN report, the leading presidential candidate in this week's Iranian presidential election, Ayatollah Ali-Akbar Hashemi-Rafsanjani, told BBC television: "It's possible that at times, Iran has not reported its activities."
But Rafsanjani, a former president, added that since the launch of an IAEA investigation into Iran's nuclear activities last year, Tehran "has made everything transparent."
Nonetheless, it's not the first time Iran has acknowledged not being fully transparent about its atomic program. Among other admissions, Iran in early 2003 acknowledged secretly importing uranium compounds, following IAEA inspections of its nuclear facility at Natanz.
Independent experts say the latest admission is again raising the question as to whether Iran is coming clean about its nuclear activities.
Shannon Kile, a senior researcher at the Stokholm Interntional Peace Institute, specializes in nuclear arms control and arm proliferation misuse.
"It calls into doubt Iran's candor in revealing what its done in the past with respect to its nuclear program, not so much that this is an activity that wasn't previously known but it simply calls into doubt whether Iran is being fully transparent about its nuclear activities in the past," Kile says.
The IAEA has not linked Iran's laboratory-scale experiments with plutonium to weapons activity. The agency says it has found no evidence that Iran is engaged in military nuclear activities. But it says its probe and inspections of Iranian nuclear sites should continue.
But analyst Kile says the IAEA report also raises doubts about the real aim of Tehran's nuclear activities.
"[Plutonium] could be used as part of a civilian nuclear fuel cycle as well, so it's not in itself a smoking gun so to speak that Iran has a nuclear weapons program. I think the problem here is that it is part of this troubling pattern of deception and concealment that's been going on in the past 18 years on Iran's part which makes many outside observers question what is the real scope and nature, the real purpose of Iran's nuclear program," Kile says.
Mohammad Saeedi, an official from the Iran Nuclear Organization, has told the semi-official Mehr Agency that "it seems that the IAEA report has a political nature."
The report is due to be delivered today to the IAEA board of governors, who are meeting in Vienna.
According to media reports, the report details inconsistencies in Iran's account on the dates of shipments of equipment for uranium enrichment. It also outlines discrepancies about when Iranian said the first meetings occurred with people connected to Pakistani nuclear scientist Abdul Qadir Khan. Khan has acknowledged selling nuclear technology to Iran and other states.
On 14 June, IAEA director Mohammad el-Baradei called on Iran to allow access to its Lavisan-Shian and Parchin sites and to provide detailed information that could shed light on unresolved issues.
He said the key remaining issues are the origin of the low and high enriched uranium contamination found by IAEA inspectors on equipment at locations in Iran, as well as Iran's centrifuge enrichment programs.
El-Baradei said Iran has provided some additional information, but said Tehran is not yet sufficiently answering the outstanding questions.
In November, Iran said it had temporarily suspended uranium-enrichment activities as a confidence-building measure. Tehran denies U.S. accusations that it is secretly producing nuclear weapons, saying its program is for peaceful purposes.