The UN atomic watchdog said Sunday that "senior officials" were flying to Tehran, the AFP news agency reported.
The International Atomic Energy Agency (IAEA) gave no further details, but on Thursday its chief Yukiya Amano held talks in Tehran on jump-starting a stalled nuclear weapons probe.
The investigation is a sticking point in ongoing talks between Iran and major powers in Vienna ahead of Tuesday's deadline to nail down a deal curbing Iran's nuclear activities in exchange for sanctions relief.
Iran and the IAEA agreed in November of 2013 on a new attempt to probe the accusations. The United States and its allies also included the investigation into a to-do list for talks with Iran meant to curb its nuclear programs in exchange for sanctions relief.
The IAEA is focused on 12 alleged activities that point to Iranian attempts to make such weapons, including suspicions that Tehran worked on the development on a nuclear payload for missiles.
The agency relaunched its probe two years ago by asking for information on less sensitive work related to nuclear arms allegedly carried out by Iran, with hopes of moving to larger issues later.
However, the IAEA has continuously reported little progress in its attempts to probe the allegations against Iran, and Amano recently said that the agency had limited progress in its inquiry into possible military dimensions of Iran's nuclear activities.
Iran rejects the IAEA's allegations that before 2003, and possibly since, it conducted research into making a nuclear weapon, saying the claims are based on bogus intelligence provided by foreign spy agencies.
Amano said after returning from Tehran that some progress had been made but that "more work" was needed, and that the IAEA aimed to finalize a report on the probe by the end of the year.
Iran and the six world powers continue intensive talks aimed at reaching a permanent nuclear deal by Tuesday, a deadline that was decided upon after the sides had already extended the talks several times.
On Friday, Iranian Foreign Minister Mohammad Javad Zarif claimed that the sides are closer than ever to reaching a nuclear agreement.
"We are ready to strike a balanced and good deal and open new horizons to address important common challenges," Zarif said.
"We have never been closer to a lasting outcome," he added.
Later Friday, a senior American official raised the possibility that the talks could be extended even beyond Tuesday if an agreement is not reached.
Asked if Iran and the major powers were "exceedingly close" to a deal whether the talks might go past that deadline, the official told reporters, "Is it conceivable that we would go past the 7th? Sure, it's conceivable … But … we want to get this done as soon as possible."