In his remarks, Khamenei — the ultimate decider in Iran — may have been seeking to increase Iranian leverage in the final phase of the talks and to counter domestic pressure on the U.S. side. Sensing that this is their last chance to influence the talks, groups such as the American Israel Public Affairs Committee and United Against a Nuclear Iran have taken out expensive newspaper ads and pressed congressional offices with demands for Iranian concessions that Tehran has made clear it will not accept.
Proponents of a possible accord concede that it will be far from perfect. Iran will be allowed to continue to enrich uranium at Natanz and will be left in possession of its other nuclear facilities, including a deeply buried site at Fordow and a heavy water reactor under construction at Arak. However, these sites will be modified in such a way that Iran would find it extremely difficult to break out and accumulate enough fuel for a bomb for at least a decade.
According to the parameters agreed to in April in Lausanne, Switzerland, the number of centrifuges at Fordow will be cut from 3,000 to 1,000 and will not be used to enrich uranium. The main enrichment plant at Natanz will mothball all but 5,000 of its 19,000 centrifuges, and Iran will reduce a stockpile of about 8,000 kilograms of low-enriched uranium to 300 kilograms — far too little to make a weapon even if further enriched.
Furthermore, the Arak facility will be reconfigured so that it produces only a minute amount of plutonium, another potential bomb fuel, and Iran will implement the Additional Protocol of the Nuclear Non-Proliferation Treaty to provide increased access to nondeclared sites suspected of illicit activity.
Those close to the talks say the final differences are over the schedule for sanctions relief and the procedure for gaining International Atomic Energy Agency access to nondeclared sites, especially military ones. Final compromises will likely await the anticipated arrival in Vienna of U.S. Secretary of State John Kerry and his diplomatic counterparts later this week.
At an event at the Atlantic Council, a Washington think tank, last month, French Ambassador to the U.S. and former nuclear negotiator Gerard Araud predicted sleepless nights and melodrama as the talks reach a climax. He also said it was likely that the June 30 deadline will slip a bit.
Critics argue that the negotiators should threaten to tighten sanctions if Iran does not accept all their demands. U.S. and European officials counter that the sanctions regime — which they say helped elect Rouhani and bring a more skillful and flexible Iranian team to the negotiating table — has peaked and cannot be augmented in a meaningful way, absent a major Iranian provocation.
"If we were to walk away or if the Congress was to make it impossible for the agreement to be implemented ... I think the international community would be pretty reluctant, frankly, to contemplate a ratcheting up further of the sanctions against Iran,” British Ambassador to the U.S. Peter Westmacott said last month.
“My sense is that we're probably not far away from the high water mark of international sanctions against the Iranian economy,” he added.