"The negotiations between Iran and the International Atomic Energy Agency have been good, constructive and are progressing," the official IRNA news agency quoted spokesman Behrouz Kamalvandi as saying.
Kamalvandi, who did not provide details, said the talks would continue tomorrow.
There was no immediate comment from the IAEA.
Chief inspector Tero Varjoranta and four experts arrived in Tehran yesterday, and held five-hours of talks with Iranian nuclear officials, led by Tehran's envoy to the IAEA, Reza Najafi, media reported.
They are expected to include long-standing allegations that Iran's nuclear work prior to 2003, or possibly since, had "possible military dimensions."
Western powers and Israel suspect that Iran's nuclear drive mask military objectives, which Tehran has repeatedly denied.