"I think the nuclear aspects of the agreements are pretty much finalised, but still there are doubts," Rafael Grossi told a nuclear panel at the Davos economic forum in Switzerland.
"This has been a long process — ongoing for more than a year — which at the moment seems to be going through a great deal of difficulty because of things that have perhaps not much to do with nuclear matters," he explained.
The IAEA and Iran said in March that they had agreed on an approach to issues that needed to be cleared to rescue the 2015 nuclear deal. The watchdog wants to clarify the presence of uranium particles at undeclared sites in Iran, Grossi said.
In 2015, Iran the P5+1 countries and the European Union signed the nuclear agreement, formally known as the Joint Comprehensive Plan of Action that stipulated gradually removing sanctions in exchange for scaling back Tehran's nuclear program.
Three years later, the United States unilaterally withdrew from the agreement and reimposed sanctions on Iran.
In April 2021, the parties to the agreement, together with the United States, began negotiations to restore the nuclear agreement by meeting for negotiation in Vienna. However, EU foreign policy chief Josep Borrell announced in March a pause in the Vienna talks due to "external factors."