GENEVA Disagreement over whether Iran has the right under international law to enrich uranium goes to the heart of the decade-old dispute over its nuclear program and has complicated diplomacy to end the standoff.
Iranian officials made clear on the third day of talks in Geneva on Friday that the Islamic state's "right" to enrich uranium must be part of any interim deal aimed at curbing its atomic activity in exchange for some sanctions relief.
But diplomats at the talks in the Swiss city later said the major sticking point may have been overcome, possibly opening the way to a long-sought breakthrough.
They said new, compromise language of a deal being discussed did not explicitly recognize a right to produce nuclear fuel by any country. "If you speak about the right to a peaceful nuclear program that's open to interpretation," a diplomat said, without elaborating.
Uranium can be used to fuel nuclear power plants - Iran's stated purpose for its atomic energy drive - but can also provide the fissile core of an atomic bomb if processed much further, which the West fears may be Tehran's ultimate aim.
Iran denies Western accusations that it is seeking the capability to build nuclear weapons.
"Iran's enrichment is non-negotiable and there is no solution without respecting Iran's right to enrichment," Foreign Minister Mohammad Javad Zarif, Tehran's chief negotiator, said this week.
The United States says no country has that explicit right under the Non-Proliferation Treaty (NPT), the 1970 global pact designed to prevent the spread of atomic bombs.
"Matters of legal theory aside, the right to enrichment has become a shorthand for the real central issue in the negotiations - whether Iran will be allowed to maintain a nuclear weapons option as part of a nuclear program under international safeguards," Gary Samore, until recently the top nuclear proliferation expert on U.S. President Barack Obama's national security staff, wrote in a Foreign Affairs article.
WHAT DOES THE NPT SAY?
Both Iran and the United States refer to Article Four of the NPT in backing up their arguments.
While recognizing every country's right to develop peaceful nuclear energy, it does not directly mention enrichment.
The NPT's opening paragraphs ban non-nuclear weapon states from developing such arms, but adds in Article Four: "Nothing in this Treaty shall be interpreted as affecting the inalienable right of all the Parties to the Treaty to develop research, production and use of nuclear energy for peaceful purposes without discrimination."
Iran argues this means it can develop the entire nuclear fuel cycle, including enrichment.
Its view wins sympathy among many non-aligned, mainly developing countries who accuse developed, affluent nations of wanting to maintain a monopoly on nuclear technology such as enrichment. They say that enrichment for peaceful purposes is implicitly recognized as a right by the NPT.
The issue is highly political for Iran, as it deems the right to produce atomic energy a matter of national pride and achievement. Decades of sanctions have made Iranians wary of relying on foreign suppliers of high technology.
But a senior U.S. official said on the eve of the Geneva talks that Washington did not believe that any country, not just Iran, had this right.
Article Four "is silent on the issue. It neither confers a right nor denies a right. So we don't believe it is inherently there", the senior Obama administration official said.
Samore said the United States and its allies have been making an additional argument: that whatever Iran's rights under the NPT might be, they have been superseded by a series of U.N. Security Council resolutions in the last seven years demanding that Iran suspend its enrichment activities.
"The resolutions are silent on how long such a suspension would last, how it would be terminated, and what kind of nuclear program Iran would be allowed post-suspension," he said.
WHY DOES IT MATTER?
Despite the Security Council demands, many experts say that it is no longer realistic to expect Iran to halt all its uranium enrichment under any settlement of the dispute as it has exponentially expanded its program in recent years.
But Western powers seem unlikely to want to officially concede on this point, and potentially allow Iran to pursue some enrichment, until they reach a final accord that offers sufficient guarantees that Iran's nuclear work is peaceful.
The aim of this week's talks between Iran and the six powers - the United States, France, Germany, Britain, China and Russia - was to hammer out an interim deal to help buy time for negotiations on a more far-reaching agreement.
"I think it is understood that Iran will have some kind of enrichment program at the end of the day," said proliferation expert Mark Hibbs of the Carnegie Endowment think tank.
However, if the initial deal grants Iran this, it "could then pocket that right, and then employ that right as an ultimate argument why it should not in the final agreement consent to any limits over its enrichment activities including number of sites, capacity, and enrichment levels", he said.
IS THERE A WAY AROUND DISPUTE?
Ahead of this week's meeting both sides signaled flexibility, raising hopes that compromise language could be found that would allow all sides to claim satisfaction.
Zarif, Iran's chief nuclear negotiator, said on November 17 that Tehran does not insist that others recognize its right to enrich, suggesting one possible way to deal with the matter.
The senior U.S. official said on Wednesday: "Do I believe this issue can be navigated in an agreement? Yes, I do."
But Iran's position seemed to harden during the course of the talks. One negotiator, Hamid Baidinejad, said on Friday: "If we want to reach a deal, our natural expectation is for them to respect our right and this to be mentioned in the deal, otherwise there is no point to negotiate."
(Additional reporting by Parisa Hafezi, Louis Charbonneau and Justyna Pawlak; Editing by Pravin Char)