It was not clear whether the extension of the meeting in Tehran meant that headway had been made towards nailing down a framework deal giving the U.N. nuclear watchdog access to sites, officials and documents for its long-stalled inquiry.
The IAEA's immediate priority is to visit the Parchin military base southeast of Tehran, where it suspects explosives tests relevant for production of nuclear weapons may have taken place, perhaps a decade ago, accusations Tehran denies.
Iran denies Western accusations that it is seeking to develop a weapons capability, saying its nuclear program is aimed only at power generation.
The Vienna-based International Atomic Energy Agency (IAEA) and Iran separately announced the continuation of their discussions that began on Wednesday. They gave no details.
The IAEA, whose mission is to prevent the spread of nuclear weapons, has been trying for a year to negotiate a so-called structured approach with Tehran on how to conduct the investigation.
"The continuation of the discussions ... has been planned for Thursday," the official IRNA news agency quoted a statement issued by Iran's supreme national security council as saying.
The IRNA report added, without giving details: "The IAEA negotiating team which arrived in Tehran on Tuesday night, held several rounds (of talks) with the Iranian team today."
In Vienna, IAEA spokeswoman Gill Tudor said: "The talks will continue on Thursday."
World powers are monitoring the IAEA-Iran talks for any signs as to whether Tehran, facing intensifying sanctions pressure, may be prepared to finally start tackling mounting international concerns about its nuclear activity.