Iran allowed IAEA nuclear officials to inspect the construction site of a heavy water reactor last week after blocking visits by the UN non-proliferation watchdog for over a year, diplomats said.
The International Atomic Energy Agency, which is expected to circulate its latest report on Iran next week, has been pushing Tehran to grant it access to the Arak site so that its experts can verify it is being designed for peaceful uses only.
The IAEA's next report will help form the basis of diplomatic talks planned for September 2, when the United States, Britain, France and Germany are expected to urge Russia and China to consider a fourth round of UN sanctions on Iran.
Iran, which says its nuclear programme is only aimed at power production and peaceful scientific research, has said the Arak complex will be geared to producing isotopes for medical care and agriculture.
But Western powers fear Iran may configure the reactor to derive plutonium from spent fuel rods as another possible source of bomb-grade fuel, besides its Natanz uranium enrichment plant, which is under daily IAEA surveillance.
"Inspectors visited (Arak) and did their job," said a senior diplomat familiar with the confidential IAEA inspections.
Diplomats also said Iran had allowed an upgrade of IAEA monitoring at the Natanz site as requested by the agency which had been finding it hard to keep track of expanding activity.
Inspectors have told the agency that containment and surveillance measures at Natanz, such as cameras and sealing, have been upgraded to the agency's current needs.
But the IAEA's next report is expected to show further growth in enrichment activities there.
The IAEA said in June that the plant was swiftly outgrowing inspectors' ability to monitor it effectively -- namely, to verify no deviations from civilian enrichment.
Some 5,000 centrifuge machines were enriching uranium then with 2,400 more being set up on the same underground production floor.
The next batch may well be refining nuclear fuel full time by now with a similar number on line for installation.