The chief UN nuclear weapons inspector said on Sunday that he was hoping for "concrete results" from his two-day visit to Iran, but said that progress "may take a while".
"We hope to have a couple of good and constructive days in Tehran," International Atomic Energy Agency chief inspector Herman Nackaerts told reporters at Vienna airport before boarding a plane to Iran.
"Importantly we hope for some concrete results from this trip... This is of course a very complex issue that may take a while," he added ahead of the visit, which was due to wrap up on Tuesday.
The visit led by Nackaerts is the second to Iran in three weeks and is seen as a key test of Iran's willingness to hold substantive talks with the IAEA about suspicious activities outlined by the agency in a November report.
Western diplomats to the IAEA said that the first visit, from January 29-31, was more of a confidence-building exercise, with the inspectors not meeting key figures in Iran's nuclear programme nor visiting any nuclear sites.
The IAEA's November report said Iran had carried out activities that were "relevant to producing" a nuclear weapon.
Since its publication the United States and the European Union have ramped up sanctions on Iran's oil sector, and speculation has grown that Iran's arch rival Israel might launch military attacks on Iranian atomic facilities.