The issue is the duration of the deal. Iran and the six world powers at the negotiating table have been far apart on this "sunset clause," with Iran wanting the comprehensive deal to last only five years, and the United States and allies wanting to stretch it for two decades or longer.
As talks have resumed in Vienna this week, Iranian, American and Russian negotiators have begun emphasizing in public comments that however difficult the terms of the deal appear, they will only last for a limited period. Then Iran will be free to pursue its nuclear ambitions, like any other signatory of the Nonproliferation Treaty, the accepted standard for nuclear activities, they say.
Iran and the six international powers -- France, Britain, Germany, Russia, China and the United States -- are seeking a deal that would lift international sanctions on Iran's economy if it accepts curbs on nuclear activities to assure that it doesn't gain a bomb-making capability.
Abbas Araqchi, Iran's deputy foreign minister, said Thursday that if Iran accepts limits on its nuclear activities, as a way of building trust with world powers, "it will only be for a specific time frame, and temporary."
"None of our commitments are for eternity, and they will not be permanent," he said in an interview with the Iranian Students News Agency.
He said negotiators are now seeking agreement on the "sunset clause." He noted that the interim agreement reached by the world powers last November specified that the final agreement would have a limited duration.
Araqchi urged Iranians not to become alarmed by reports that its negotiators are making concessions, since any deal would be temporary.
A senior Obama administration official said Thursday that whatever terms Iran accepts, it will be free to choose its own path once the deal lapses.
"What choices they make after they get to normal -- that is, after a long duration of an agreement, when they will be treated as any other nonnuclear weapons state under the [Nonproliferation Treaty] -- will, of course, be their choice," the U.S. official said.
The official declined to be identified under ground rules set by the administration.
Ray Takeyh, an Iran specialist with the Council on Foreign Relations, said this new emphasis could reflect a decision by the Iranian team that they can make concessions on key points provided they can win agreement that the deal will only last a limited time.
If they could nail down a favorable sunset clause, Iranian officials would be able to tell their public that they had won two big concessions -- a lifting of restrictions on Iran's nuclear advancement, plus elimination of the international sanctions that have been crushing the country's economy.
From their perspective, this makes sense, Takeyh said. "Once the deal is over, they're off to the races," he said.
Still, agreement on the duration could be difficult to reach. Influential U.S. allies Israel and Saudi Arabia may resist a deal lasting only a decade, as could U.S. lawmakers, who could still sink any deal.
Some Iran specialists believe that Iran's supreme leader, Ayatollah Ali Khamenei, who is 76, might not accept a decade-long deal.
The talks began a sixth round on Wednesday and are expected to continue at least until July 20. The two sides reached an impasse on key issues at a meeting in May, and remained far apart on key issues.
On Friday, Araqchi and Helga Schmid, the European Union's deputy foreign policy chief, met for another try at drafting the text of an agreement.
Earlier, a U.S. delegation led by Deputy Secretary of State William J. Burns met with an Iranian team led by Araqchi.
Special correspondent Ramin Mostaghim in Tehran contributed to this report.
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