U.N. nuclear agency

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.

That key condition is significantly reducing its stocks of enriched uranium that could be used for atomic weapons.

Iran's failure to comply would have severely undermined the negotiations, which are aimed at curbing the Iranians' nuclear program for a decade in exchange for tens of billions of dollars in relief from international economic sanctions

The State Department announced the extra days of talks only hours before the expiration of the target date for their completion.