A cooperative atmosphere is prevailing in the nuclear talks with Iran, according to the International Atomic Energy Agency (IAEA) head Rafael Grossi, who traveled to Tehran on Friday after the discovery of highly enriched uranium in the Islamic Republic, while talks were ongoing with Iran on two sets of important issues including the science sector, and there was "great expectation" about the process.

Rafael Grossi began meetings, in Tehran on Friday, that diplomats said were meant to push Iran to cooperate with an IAEA investigation into uranium traces found at undeclared sites that had been enriched close to nuclear-weapons grade.

"Globally speaking, there are two sets of issues that are important. Clearly, there is great expectation about our joint work in order to move forward related to issues that Iran and the agency are working on, and to clarify and to bring credible assurances about the nuclear program in Iran," Grossi told reporters in Tehran.

"The second set of issues, which are very important, has to do with scientific and technical cooperation we have and will continue to have with Iran," he said, speaking alongside Mohammad Eslami, head of the Atomic Energy Organization of Iran.

Grossi said the talks were taking place in an "atmosphere of work, honesty and cooperation."

His visit comes amid contacts with Tehran on the origin of the uranium particles enriched to up to 83.7% purity, very close to the 90% threshold for weaponisation, at its underground Fordow enrichment plant, according to a report by the U.N. nuclear watchdog accessed by Reuters.

Eslami said on Wednesday that the Islamic Republic was enriching uranium up to 60% fissile purity.