The head of the UN atomic agency called Monday on Iran to allow immediate access to the Parchin military base where it suspects nuclear weapons research took place.
Yukiya Amano said that this should be granted "without further delay" and without waiting for stalled talks to conclude an agreement on investigating other alleged "weaponisation" activities.
"I request Iran once again to provide access to the Parchin site without further delay, whether or not agreement has been reached on the structured approach," Amano told the International Atomic Energy Agency board of governors meeting.
"Providing access to the Parchin site would be a positive step which would help to demonstrate Iran's willingness to engage with the Agency on the substance of our concerns," he said, according to the text of his remarks at the closed-door gathering.
Iran has refused to give the IAEA access to sites, documents and scientists involved in what the agency suspects were efforts, mostly in the past but possibly ongoing, to develop nuclear weapons.
More than a year of meetings, the latest on February 13 in Tehran, have failed to agree on a so-called "structured approach" deal to address all the allegations.
Amano said Monday that "negotiations must proceed with a sense of urgency" and that he "would like to report real progress by the next meeting of the next (IAEA) board meeting in June."
Tehran says that the IAEA's conclusions about the "possible military dimensions" of its programme are based on flawed information from Western and Israeli spy agencies, information that it says it has not been allowed to see.
It denies working or ever having worked on nuclear weapons and says that no nuclear activities have taken place at the Parchin military base near Tehran and that therefore the IAEA has no right to conduct inspections there.
The IAEA visited the site twice in 2005 but says that since then it has obtained additional indications of activity there that make it want to go back.
The agency also conducts regular inspections of Iran's declared nuclear sites and its quarterly reports routinely outline advances in its atomic programme in spite of UN Security Council resolutions calling for a suspension.