Since the International Atomic Energy Agency "is not yet in a position to clarify some important outstanding issues after two and a half years of intensive inspections and investigation, Iran's full transparency is indispensable and overdue," the IAEA said in a confidential report obtained by

The IAEA has since February 2003 been investigating Iran on US charges that the Islamic republic is using an allegedly peaceful civilian nuclear program as a cover for secret development of atomic weapons.

The report said Iran was still refusing to allow IAEA inspectors visits to the Parchin military site, where it has been to only one part of what is a huge complex where weapons explosive work is suspected, and to the Lavizan site in Tehran where there is suspicion that enrichment work was carried out and where the soil has been razed.

There are also open questions about Iran's work with both uranium and plutonium, the two raw materials for atom bombs, as well as with centrifuges that refine uranium and with other sensitive materials Iran has tried to obtain on the black market, such as beryllium which can be used to make the trigger for nuclear weapons.

"The burden of proof is back to Iran," a senior official close to the IAEA said.