A look at Iran's nuclear assets:
At the oldest and largest uranium enrichment site, in Natanz, about 260 kilometers (160 miles) southeast of Tehran, an underground hall can hold up to 50,000 centrifuges. Iran says they only are used for energy, science or medicine. But they also could be used to churn out the fissile core of a nuclear warhead.
A second site, Fordo, was built into a mountainside south of Tehran. Iran acknowledged its existence six years ago after the United States shared intelligence about it with allies. The hardened underground bunker is thought to be resistant, if not impervious, to air attack.
Fordo is large enough to hold around 2,000 centrifuges. Some experts say its design hints more at a nuclear weapons-making facility than one for peaceful uranium enrichment.
Before Iran temporarily froze its atomic activities in late 2013 and embarked on the present nuclear negotiations, it was enriching uranium up to 20 percent at Fordo. That is only a technical step from the level used in warheads.
ARAK HEAVY WATER REACTOR
Iran says that the nearly completed reactor, 240 kilometers (145 miles) southwest of Tehran, is meant for making medical isotopes and research. Many other countries say it is poorly designed for that purposes
If completed as planned, the reactor would produce enough plutonium for several bombs a year. Iran does not currently have any known facilities to process the plutonium for military uses.
Iran has a research reactor in Tehran that mainly produces isotopes for cancer treatment, and Bushehr nuclear power plant, which began full operations in early 2013.
The main uranium mine is at Saghand, in the central Yazd province. Iran operates a smaller mine and processing facility in Gachin, near the port city of Bandar Abbas on the Persian Gulf.
There are plans for a plant at Ardakan, about 500 kilometers (300 miles) southeast of Tehran. It would process uranium ore into yellowcake, the basic feedstock for uranium enrichment.
The Isfahan Uranium Conversion Facility reprocesses yellowcake into a gas, which is fed into centrifuges for enrichment. A separate facility in Isfahan, about 250 kilometers (150 miles) south of Tehran, produces nonenriched fuel for the planned Arak reactor.