The UN's atomic watchdog and Iran held "intense discussions" in Tehran on Monday about the possible military dimensions of the country's nuclear program, AFP reported, citing an Iranian official.
The meeting was held as parallel talks were underway in Vienna between Iran and world powers on the eve of a deadline for an accord on the country's suspect nuclear drive.
It comes just days after the head of the UN's International Atomic Energy Agency (IAEA), Yukiya Amano, visited Tehran to try to advance Iran's negotiations with world powers over its nuclear program.
The IAEA suspects Iran carried out research at least until 2003 on developing nuclear weapons.
It is seeking access to scientists who might have been involved, as well as documents and the sites at which any such activities took place.
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.
Following his trip to Iran last Thursday, Amano said the talks touched on the increased role the IAEA would have after a deal with world powers to curb the country's nuclear program.
Iran's Atomic Energy Organization said that on Monday the IAEA delegation held "intense negotiations" with officials from the Islamic Republic.
"The five-member delegation are discussing details of the understanding reached in Yukiya Amano's recent trip to Tehran," said spokesman Behrouz Kamalvandi, according to AFP.
"They visited officials from the Atomic Energy Organization, Supreme Security Council and the agency's permanent envoy to Iran," he said, quoted by state media.
The IAEA visit to Tehran comes as 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 Monday, Iran demanded yet another concession in nuclear talks – this time, to end the UN arms embargo on the Islamic Republic.
With tensions high and progress still incremental just one day before that deadline, Iran announced that it is working with the U.S. and UN to end the arms embargo – a proposal that the U.S. is likely to categorically reject as part of efforts to reduce Iran's firepower.