UN atomic watchdog chief Rafael Grossi on Tuesday decried "completely unsatisfactory" cooperation from Tehran after returning from Iran where he urged leaders to adopt "concrete" measures to address concerns over its nuclear programme.

Grossi's visit came at a time of heightened regional tensions and with his International Atomic Energy Agency (IAEA) criticising Iran for lack of cooperation on inspections and other outstanding issues.

"The present state is completely unsatisfactory for me. We are almost at an impasse and this needs to be changed," Grossi told reporters at the airport in Vienna, where the IAEA is based.

He said there was no "magic wand" to solve a "very, very complex set of issues," while he pressed the Islamic republic to "deliver very soon."

"But of course, for me and also I would say for the international community, there is a need to have some results sooner rather than later," he said.

Earlier on Tuesday, at a news conference in the Iranian city of Isfahan, Grossi said he had proposed to Iranian officials that they "focus on the very concrete, very practical and tangible measures that can be implemented in order to accelerate" cooperation.

Grossi held talks with senior Iranian officials including Atomic Energy Organisation's head Mohammad Eslami and spoke at Iran's first International Conference on Nuclear Science and Technology held in Isfahan.

Grossi insisted on the need to "settle differences" on the nuclear issue while the Middle East was going through "difficult times", particularly the war between Israel and the Iran-backed Palestinian militant group Hamas in the Gaza Strip.

"Sometimes, political conditions pose obstacles to full-fledged cooperation" between Iran and the international community, Grossi told reporters.

Grossi said a March 2023 agreement with Iran was "still valid" but required more "substance."

The agreement was reached during Grossi's last visit to Iran and outlined basic cooperation measures including on safeguards and monitoring.

The IAEA chief said, however, that there had been a "slowdown" in the agreement's implementation, including Iran reducing the number of inspections and withdrawing the accreditation of a group of IAEA experts.

Iran suspended compliance with a landmark 2015 deal setting caps on nuclear activities after the United States unilaterally withdrew from the agreement in 2018 and reimposed sweeping sanctions. (AFP)