Yukiya Amano

Such sampling of soil, air or dust from equipment is usually done by the International Atomic Energy Agency's (IAEA) own experts. But IAEA chief Yukiya Amano confirmed that Iranians carried out that part of the probe at Parchin, where the agency suspects that explosive triggers for nuclear weapons might have been tested in the past.

Diplomats say Iran insisted on the compromise as a condition for any probe of Parchin.

Yukiya Amano, head of the U.N.'s International Atomic Energy Agency, said his meetings with Iranian President Hassan Rouhani resulted in a "better understanding on some ways forward," but that "more work will be needed."

The formulation of his statement was similar to previous ones issued by the IAEA, which has struggled for nearly a decade to resolve its concerns.

Amano's trip Thursday to Tehran was significant because it represented his last chance to secure access and cooperation before a July 7 deadline for a long-term nuclear agreement between Iran and world powers.