The head of the U.N. nuclear watchdog urged Iran on Monday to give his inspectors immediate access to the Parchin military site, where they suspect explosives tests relevant to nuclear weapons development may have taken place.

Yukiya Amano, director general of the International Atomic Energy Agency, told the IAEA's 35-nation board that he was "unable to report any progress" in efforts since early 2012 to clarify concerns about Iran's nuclear program.

The Vienna-based IAEA has been trying for more than a year to persuade Iran to cooperate with a long-stalled agency investigation into suspected atom bomb research by the Islamic state, which denies any such activity.

Amano said the agency remained committed to engaging in constructive dialogue with Iran, but that negotiations must proceed with "a sense of urgency and a focus on achieving concrete results" soon.

The IAEA's priority is to be able to inspect Parchin, a sprawling site southeast of the capital Tehran, where it believes Iran built an explosives chamber to carry out tests, possibly a decade ago. Iran denies this.

"Providing access to the Parchin site would be a positive step which would help to demonstrate Iran's willingness to engage with the agency on the substance of our concerns," Amano said, according to a copy of his speech.