Iran and six world powers began two days of expert-level talks Monday to iron out details in implementing a landmark accord for Tehran to curb its disputed nuclear program.
Officials from Iran and the United States, France, Germany, Britain, China and Russia met at the Vienna headquarters of the U.N. nuclear agency, which will play a central role in verifying that Tehran carries out its part of the interim deal.
The International Atomic Energy Agency said it would have "some involvement" in the closed-door discussions, which are expected to continue on Tuesday.
Iranian officials are due to meet just with IAEA officials on Wednesday.
The outcome of the meetings are expected to determine when Iran stops its most sensitive nuclear activity and when it gets the respite in sanctions it has been promised in return.
In a major diplomatic breakthrough last month, Tehran committed to limit uranium enrichment for six months and not to make further advances at its Fordo, Natanz and Arak facilities.
Iran will also neutralize its stockpile of medium-enriched uranium - which is easy to convert to weapons-grade - and allow more intrusive IAEA inspections.
The preliminary accord is seen as a first step towards resolving a decade-old standoff over suspicions Iran might be covertly pursuing a nuclear weapons "breakout" capability.
IAEA inspectors visited the Arak heavy water production facility for the first time in two years on Sunday.